In 1827, Karl Ernst von Baer sat down in Königsberg with a microscope and the ovary of a dog. For more than a century, anatomists had argued about where a mammal begins. Some held that a complete miniature body lay ready-made inside the sperm and needed only feeding. Others gave the whole credit to the mother. Neither side had ever seen the thing they were arguing about.
An earlier anatomist, Reinier de Graaf, had noticed small fluid-filled blisters on the surface of the ovary and had concluded that these blisters were the eggs. They could be seen without a microscope, which should have been a warning. They were far too big.
Von Baer opened one of those blisters. Floating in the fluid inside was a yellowish speck, small enough to be missed altogether. He lifted it onto the microscope stage and looked down at a single rounded cell with a nucleus at its centre. The blister was not the egg. The egg lay inside the blister, and it was one cell. Fig. 13.0-1 shows what lay in front of him.
That one observation settled the argument. A dog, a cow, a human being — each begins as a single cell released from an ovary, barely visible to the unaided eye, carrying half of the instructions the new individual will need. The other half arrives separately, and this chapter is the story of how the two meet.
Q1. Von Baer found the egg inside a fluid-filled sac. What is that sac called, and what happens to it after the egg leaves? Q2. Millions of sperms set out towards one egg. Why does only one of them ever get in? Q3. The testes hang outside the body in a thin sac of skin instead of sitting safely inside. Why would a body be built that way? Q4. The mother’s blood and the unborn baby’s blood never mix. So how does food and oxygen get from one to the other? Q5. Why do some twins look exactly alike, while others look no more alike than any brother and sister?
13.1 Reproduction In General
An animal can live a full life without ever reproducing. It feeds, it breathes, it grows, and in time it dies. Nothing inside its own body depends on its producing young.
Yet if every individual lived that way, the kind itself would disappear within a single lifetime. Reproduction is the one life process that works for the species rather than for the individual. It is the process by which living organisms produce new individuals of their own kind, and those new individuals are themselves built to do the same in their turn.
Reproduction The process of formation of new individuals by sexual or asexual means, the new individuals being able to repeat the process in their own turn.
Living things do not all go about it in the same way. Bacteria and yeast make new individuals by simply dividing or by growing outgrowths from the body. Many plants can raise a whole new plant from a stem, a root or a leaf. Human beings do none of these things — in humans, reproduction takes place only by the sexual method.
Behind this variety lie just two broad patterns, and every organism you will meet in this chapter uses one or the other.
13.1.1 Asexual Reproduction
Suppose an organism could build a new individual out of a piece of itself, with no partner involved anywhere in the process. That is exactly what happens in asexual reproduction — reproduction by a single parent, without any sex cells joining.
The portion of the parent that becomes the new individual is its reproductive unit. This unit may be as large as the entire parent body or as small as a fragment of it, and the two examples below sit at the two ends of that range.
Fission In Amoeba
Amoeba is a single-celled organism found in pond water. Here the reproductive unit is the whole animal. The parent cell divides into two daughter cells, and each daughter grows and lives independently.
This splitting of one cell into two is called binary fission. Notice what it means: once the division is over, there is no parent left standing beside the offspring. There are simply two new individuals where there was one.
Budding In Hydra
Hydra is a small freshwater animal only a few millimetres long, and it uses a far smaller unit. A swelling appears on the side of its body and grows outward, developing its own mouth and tentacles.
This outgrowth is a bud, and the process is called budding. When the bud is large enough it breaks away and lives on its own. Here, unlike in fission, the parent survives the process and can bud again. Both methods are drawn side by side in Fig. 13.1-1.
Binary Fission And Budding In binary fission the parent divides into two equal parts and ceases to exist as a separate individual. In budding an unequal outgrowth separates from the parent, and the parent continues to live.
Because no partner has to be found and no sex cells have to meet, asexual reproduction is quick. In favourable conditions one organism can fill a pond with offspring in a matter of days.
There is a second consequence, and it matters more than speed. Every offspring is built from the parent’s own body, so every offspring carries the same genetic material as the parent. The young are copies.
Why Farmers Plant Cuttings Sugarcane is grown from pieces of stem, and banana from suckers thrown up at the base of the parent plant. Both are asexual methods, so every plant in the field carries the same genetic material as the parent it came from. The grower gains a crop of even quality that ripens together. The same sameness is a risk: a disease that can attack one plant can attack every plant in that field.
13.1.2 Sexual Reproduction
Most large and complex organisms, human beings among them, cannot raise a new individual from a fragment of the body. They use sexual reproduction, in which two special cells, one from each of two parents, fuse to start the new individual off.
These special cells are the gametes, or sex cells. In animals the male gamete is the sperm and the female gamete is the egg, also called the ovum. Both are single cells, and in human beings both are microscopic.
A sperm on its own cannot become a baby, and neither can an egg. Each carries only half of the instructions the new individual needs. The two must meet and fuse — a process called fertilisation — and the single cell produced by that fusion is the zygote.
The zygote is where the new individual actually begins. It divides again and again, and over the following weeks and months develops into the offspring. FC 13.1 sets out the whole sequence in order.
Where You Are On The Ladder Zoom in: the sperm and the egg are single cells. Zoom out: the zygote they form becomes an embryo, then a whole organism, and that organism joins a population of its own kind.
Because the offspring receives instructions from two different parents, it is a copy of neither. Brothers and sisters raised from the same two parents resemble one another, and yet no two of them are identical.
This built-in variation is the real advantage of sexual reproduction. The price paid for it is time and effort: two parents must be brought together, gametes must be produced and must meet, and the offspring are fewer.
13.1.3 Asexual And Sexual Reproduction Compared
The two patterns differ at almost every point, and the differences all follow from one root cause — whether gametes are involved. Table 13.1 sets them against each other.
Table 13.1 — Asexual And Sexual Reproduction Compared
| Gametes |
No gametes are formed |
Gametes are formed |
| Genetic material |
No mixing of genetic material, so there is little or no variation among the offspring |
Genetic material of two parents mixes, so variation among the offspring increases |
| Number of offspring |
Normally more |
Fewer |
| Parents involved |
Only one parent |
Usually two parents, one male and one female |
| Speed |
A rapid process in favourable conditions |
A slower process |
13.1.4 Secondary Sexual Characters
In a great many animals you can tell the male from the female at a glance, without knowing anything at all about their reproductive organs. A lion carries a mane and a lioness does not. A peacock spreads a long train of coloured feathers, while the peahen has none. A cock has a tall red comb and sweeping tail feathers that a hen lacks, and a bull is heavier and more heavily muscled than a cow.
These outward differences between the two sexes are called secondary sexual characters, or secondary sexual characteristics. They are secondary because they are not the organs that make gametes and they take no direct part in fertilisation.
Their work is to advertise. They allow members of a species to recognise the opposite sex and to be attracted to it, which is what brings two parents together in the first place.
The signals are not always visual. Body odours, calling sounds, skin colouration, body contour and patterns of behaviour all serve as sex clues in one animal or another.
In Human Beings
Human beings show such differences too, and they appear during puberty rather than at birth. The characters usually listed are these.
- Distribution of hair on the body and face, including the beard and moustache in males
- Breasts in females
- Muscularity, the build being stronger in males
- Skeletal structure, including the broader hips of females
- Psyche and behaviour
- A deeper voice in males
Some of these differences are set out in the outlines in Fig. 13.1-2. Like the mane of a lion, none of them produces a gamete or carries one anywhere. What produces the gametes is the subject of the next section.
Look at a photograph of a peacock and a peahen, or at a cock and a hen if there are birds near you. List two differences you can see between the two, and then decide which of those differences could help one bird find the other.
13.2 Reproduction In Humans
Human beings reproduce only by the sexual method. That single fact fixes what the body must be able to do: it must make gametes, it must bring the two kinds of gamete together, and it must shelter and feed the new individual until it can survive outside.
No one organ can do all three. The reproductive system is therefore a set of organs with a division of labour, and biologists sort them into two groups by the job each one performs.
13.2.1 Primary And Accessory Reproductive Parts
The primary reproductive parts are the gonads — the organs that actually manufacture the gametes. In males the gonads are the testes, which produce sperms; in females they are the ovaries, which produce eggs. There is only one pair of gonads in each body, and without them nothing else in the system has any purpose.
The accessory reproductive parts are everything else: the ducts, the glands and the structures that carry the gametes, bring them together for fertilisation, and then house and nourish the developing young until the baby is born. They handle no gamete manufacture at all, but transport, delivery and shelter.
Secondary sexual characters belong in neither group. A beard makes no gamete and carries none, so it is not a reproductive part, even though it appears only because the reproductive system has begun to work. Table 13.2 keeps the two true groups apart.
Table 13.2 — Primary And Accessory Reproductive Parts
| What they are |
The gonads only — testes in the male, ovaries in the female |
All the ducts, glands and other structures of the system |
| Chief work |
Production of the gametes |
Transfer of the gametes, and shelter and nourishment of the young |
| Number |
One pair in each body |
Several |
| If they are absent |
No gametes are formed at all |
Gametes may be formed but cannot meet |
13.2.2 The Male Reproductive System
The male system has one task above all others: to make sperms in very large numbers and deliver them into the female body. Its organs, shown in Fig. 13.2-1, are the following.
- The testes, one pair, which produce the sperms
- The sperm duct or vas deferens, one from each testis, which carries the sperms away
- The accessory glands — the seminal vesicles, the prostate gland and the bulbo-urethral glands — which contribute the fluid part of the semen
- The penis, which transfers the sperms into the female vagina
- The urethra, contained inside the penis, which conveys the sperms received from the sperm ducts
13.2.2.1 The Testes
The two testes are oval organs, and they are not held inside the body cavity at all. They hang outside it, in a thin-walled sac of skin called the scrotum.
This is a strange arrangement for organs so easily injured, and it needs explaining. Sperms will not mature at the temperature of the body interior. They are produced at a temperature about 2 to 3 °C lower than the rest of the body, and a slightly higher temperature simply stops them maturing. Slung outside in a separate sac, the testes escape too much body heat.
The scrotum does more than hold them there. When the surrounding air is hot, its skin loosens and the testes hang lower, away from the warm body wall. When it is cold, the skin contracts into folds and draws them closer to the body for warmth. The sac keeps its contents within a narrow band of temperature by changing the distance between them and the body.
Q3. The testes hang outside the body in a thin sac of skin instead of sitting safely inside. Why would a body be built that way?
A. Because sperms cannot mature at the body’s own internal temperature. They need conditions 2 to 3 °C cooler, and only a sac slung outside the body can provide them.
Descent Of The Testes
The testes do not begin life outside the body. In the embryo they lie within the abdomen, and they descend into the scrotum shortly before birth.
Occasionally a testis fails to descend. Kept at the abdomen’s temperature, it cannot produce mature sperms, and the result is sterility — the inability to produce sperms. The descent is not an accident of development; it is the step that makes sperm production possible.
Structure Of The Testis
Each testis is enclosed in a capsule, and this capsule sends partitions inwards that divide the organ into around 250 lobules, or segments. Fig. 13.2-2 shows the arrangement. Each lobule contains two things.
- Seminiferous tubules (semini, sperm; ferous, bearing) — fine coiled tubes in which the sperms are actually produced. The process of forming sperms is called spermatogenesis.
- Interstitial cells (interstitial, filling in between) — packing tissue lying in the spaces between the coils of the tubules. These are also called Leydig cells, and they produce the male sex hormone testosterone.
Notice that the two jobs are separated. One set of cells makes the gametes; a different set, tucked into the gaps, makes the hormone.
The Testis On The Scale Ladder Zoom in: interstitial cell (a cell) → seminiferous tubule (a tissue). Zoom out: lobule → testis (an organ) → male reproductive system (an organ system).
The Path Out Of The Testis
The sperm-producing cells of the seminiferous tubules keep multiplying and turning out sperms. The mature sperms pass first into a small network of tubes inside the testis.
From that network arise 12 to 14 fine efferent ducts. These join a small tubular knot, the epididymis (epi, above; didymos, testicle), which fits like a cap over the upper pole of the testis and then runs down along its side to the back.
The epididymis looks compact, but internally it is a single coiled tube about 6 metres long. Sperms are stored in it for some days, and during that stay they complete their maturing and become motile — that is, capable of moving.
From the back of the epididymis a distinct tube, the sperm duct or vas deferens, arises and leaves the scrotum. The whole route from tubule to urethra is set out in FC 13.2.
Epididymis A single tube about 6 metres long packed into a small knot → sperms take days to pass through it → they have time to mature and become motile before they leave.
13.2.2.2 The Sperm Ducts
The sperm duct from each testis travels upward out of the scrotum and into the abdomen, passing through a short passage in the abdominal wall called the inguinal canal. This canal is not built for the duct. It is the passage through which the testis itself descended before birth, taking its duct, blood vessels and nerves along with it.
Inside the abdomen, the two sperm ducts loop over the ureter of their own side, come together behind the urinary bladder, and join the median duct — the urethra. Just before this junction, the duct from each seminal vesicle joins the sperm duct, and the short common tube so formed is the ejaculatory duct.
From here onward there is a single shared passage. The urethra carries semen and urine alike, though never at the same moment.
Ureter And Urethra The ureter carries urine from a kidney down to the urinary bladder. The urethra carries urine out of the bladder to the exterior, and in the male it carries semen as well.
Inguinal Hernia The inguinal canal is a weak point left behind in the abdominal wall after the testis has passed through it. When pressure inside the abdomen rises — during heavy lifting, for instance — a loop of intestine can be pushed through the canal and bulge into the scrotum. This is an inguinal hernia, the most common type of hernia, and it is repaired surgically by closing the gap.
13.2.2.3 The Male Accessory Glands
Sperms leaving the epididymis are alive but sluggish, and they have a long journey ahead of them through the female body. Three sets of glands add fluid to them along the way, and each addition solves a different problem.
- The seminal vesicles are a pair of lobulated glands lying between the back surface of the urinary bladder and the rectum. A duct from each joins the corresponding sperm duct just before it unites with the urethra. Their secretion serves as the medium in which the sperms are transported, and the sperms, sluggish until now, become active in it. The mixture of this fluid and the sperms is a milky fluid, the semen.
- The prostate gland is a bilobed structure that surrounds the urethra close to where it leaves the bladder. It pours an alkaline secretion into the semen as the semen passes through. The contents of the vagina are acidic, and this alkali neutralises that acid — without it the sperms would not survive long after delivery.
- The bulbo-urethral glands, also called Cowper’s glands, are two small ovoid glands opening into the urethra just before it enters the penis. Their secretion acts as a lubricant.
13.2.2.4 The Penis And The Semen
The penis lies in front of the scrotum and is cylindrical in shape. It is the passage for both semen and urine, and it is the organ that transfers the sperms into the female vagina.
It is a highly vascular organ, built largely of erectile tissue riddled with vascular spaces, or sinuses. Under sexual stimulation, blood flows into the penis in large amount and fills these spaces. As they expand they press on the veins, so blood cannot drain away as fast as it arrives, and the organ becomes rigid and erect. This condition is called erection, and it is what makes the transfer of semen possible.
The semen delivered is the mixture of sperms with the secretions of the seminal vesicles, the prostate and the bulbo-urethral glands. It is a milky fluid. A single ejaculation contains on average 2 to 3 ml of it, carrying between 200,000,000 and 400,000,000 sperms — hundreds of millions of gametes for the one that will be used.
13.2.3 The Female Reproductive System
The female system has to do far more than the male one. It must produce eggs, receive sperms, provide the place where fertilisation happens, and then hold and feed the developing young for months. Its organs, shown in Fig. 13.2-3 and Fig. 13.2-4, are the following.
- A pair of ovaries
- A pair of oviducts, or Fallopian tubes, which convey the egg released from the ovary
- A sac-like, pear-shaped uterus, in which the embryo grows and develops
- A vagina
- The vulva, the outermost part
13.2.3.1 The Ovaries
The two ovaries are small ovoid bodies, and it is their outer, peripheral part that produces the ova, or eggs. Normally one egg matures and is released each month, from one ovary at a time.
An egg does not develop bare in the tissue of the ovary. A maturing egg is held inside a small sac of cells called the follicle, which protects and feeds it. As the egg grows larger, the follicle enlarges around it and fills with fluid, and is then called the Graafian follicle. When it is fully ripe, it bulges out on the surface of the ovary like a blister.
The whole process by which the ova-producing cells of the ovary give rise to a mature ovum is called oogenesis. Its counterpart in the male, as you have seen, is spermatogenesis.
Then the ripe follicle bursts. This rupture, releasing the egg from the ovary, is ovulation, and the stages leading up to it are drawn in Fig. 13.2-5.
Ovulation The rupture of the ripe Graafian follicle and the release of the egg from the ovary.
The released egg does not travel far on its own. It is picked up by the fimbriae, finger-like ciliated processes of the funnel of the oviduct waiting close to the ovary.
Meanwhile the emptied follicle does not simply vanish. Its remnant persists in the ovary and turns into a yellow mass called the corpus luteum (corpus, body; luteum, yellow). This yellow mass is a gland, and it secretes two hormones — oestrogen, which the follicle had also been secreting before ovulation, and progesterone.
Q1. Von Baer found the egg inside a fluid-filled sac. What is that sac called, and what happens to it after the egg leaves?
A. The fluid-filled sac is the Graafian follicle. After the egg is released at ovulation, what remains of the follicle becomes the corpus luteum, a yellow hormone-secreting mass that stays in the ovary and produces oestrogen and progesterone.
Ovum And Egg
In everyday speech the two words are used as though they meant the same thing, and in biology they do not quite. The ovum is the female gamete itself — the proper egg cell, with no extra parts.
A hen’s egg shows the difference plainly. The ovum is only the central yellow sphere. The nutritive albumen, the white, and the shell around it are all added later, as the ovum passes down the oviduct. So every ovum is an egg, but not every egg is just an ovum.
13.2.3.2 The Oviducts
The two oviducts, also called Fallopian tubes or uterine tubes, are each about 12 cm long. Near the ovary of its own side, each oviduct opens out into a funnel-shaped mouth, the oviducal funnel, and it is the cilia lining this funnel that pick up the released ovum and push it into the tube.
The ovum has no means of moving itself, so the tube must move it. Waves of muscular contraction pass along the oviduct wall — the same peristalsis that moves food along the alimentary canal — and these waves carry the egg steadily down towards the uterus.
13.2.3.3 The Uterus
The uterus is a hollow, pear-shaped, muscular organ, roughly 7 cm long and 5 cm across, lying in the pelvic cavity between the urinary bladder and the rectum. This is the organ in which the embryo will grow.
It has two regions. The upper, wider portion receives the two oviducts, one at each side. The lower part is small and constricted, and this narrow neck is the cervix.
The thick muscular wall is not decoration. It stretches enormously as the young one grows, and at the end of pregnancy the same muscle contracts powerfully to push the baby out.
13.2.3.4 The Vagina
The vagina is a muscular tube, 10 to 15 cm long, running from the lower end of the uterus to the outside of the body. It receives the penis during copulation, and it is here that the semen is deposited.
Its wall is remarkably elastic, and that elasticity has a second use: the vagina is the passage through which the baby leaves the mother’s body at birth.
The opening of the vagina in young females is partially closed by a thin membrane called the hymen. It varies a great deal from person to person, and it is frequently ruptured in childhood by strenuous physical exercise or by disease.
13.2.3.5 The Vulva
The external female genitalia together are called the vulva. It contains the separate openings of the urethra and the vagina, which stay independent of each other throughout — unlike the male system, where the urethra is a shared passage.
At the sides lie two small fleshy folds, the labia minora or lesser lips, and these are hidden by larger folds carrying hair, the labia majora or greater lips. These folds correspond to the scrotum of the male.
In the uppermost angle of the vulva, in front of the urethral opening, lies a small erectile structure, the clitoris. It is highly sensitive, and it is the equivalent of the male penis.
13.2.4 Puberty And The Role Of Sex Hormones
All the organs described above are present in a child, but none of them is working. They are brought into action at a particular stage of life, and the switch that turns them on is hormonal.
Puberty The period during which the immature reproductive system of boys and girls matures and becomes capable of reproducing.
In girls, the ovaries begin to function at around 10 to 14 years of age, and start releasing ova. The first outward sign, at about the age of ten, is the enlargement of the breasts.
At about the same time as the first ovulation, the ovary begins releasing female sex hormones into the blood stream. These hormones, the oestrogens, bring about the growth of the mammary glands, a widening of the hips, the growth of hair in the pubic region and the armpits, and an increase in the size of the uterus and the vagina. These changes are the secondary sexual characteristics of the female.
Progesterone, secreted by the corpus luteum, has a different task altogether. It does not shape the body outwardly; it prepares the lining of the uterus to receive an embryo.
In boys, puberty comes at around 11 to 15 years of age, the first sign at about eleven being the enlargement of the testes. The testes begin producing testosterone, and this hormone brings about the deepening of the voice, the growth of the testes and the penis, and the growth of hair on the chest, in the pubic region and the armpits, and on the chin and the upper lip.
So the outward differences you met in Section 13.1.4 are not separate from the reproductive system at all. They are the visible signal that the gonads have started work.
13.3 The Menstrual Cycle
The ovaries do not work steadily throughout a woman’s life, and they do not work steadily even during the years when they are active. They work in a repeating cycle.
The reproductive period of the human female runs from about the age of 13 years to about 45 to 50 years. Throughout that period one characteristic event repeats itself almost every month: a discharge of blood from the uterus, the menstrual flow. Pregnancy interrupts it, and it stops for good at the end of the reproductive period.
The two ends of this period have names of their own. Menarche (arche, beginning) is the onset of menstruation in a young female, at about the age of 13 years. Menopause (pause, stop) is the permanent stoppage of menstruation, at about the age of 45 years.
Between those two events the cycle turns over and over. One complete turn is a menstrual cycle, and it is counted from the day the flow begins to the day the next flow begins, 28 days later. The starting point is chosen for a practical reason: the beginning of the flow is the one event in the whole cycle that can be seen and dated exactly.
Menstrual Cycle The cycle of changes in the ovary and in the lining of the uterus that repeats about every 28 days during the reproductive period, counted from the onset of one menstrual flow to the onset of the next.
The cycle has four phases, and they run in this order.
- Menstrual phase
- Follicular phase
- Ovulatory phase
- Luteal phase
Two organs are changing at the same time all the way through. The ovary is preparing and releasing an egg, while the uterus is preparing a lining to receive it. Fig. 13.3-1 shows the two sets of changes drawn one above the other against the same scale of days, and FC 13.3 sets out the cycle as a whole.
13.3.1 The Menstrual Phase
The cycle opens with the phase that gives it its name. The menstrual phase lasts for 3 to 5 days, and during it blood is discharged from the uterus through the cervix and the vagina.
The blood does not come from a wound. The innermost lining of the uterus, called the endometrium, has been thickened and packed with blood vessels during the cycle before. When it is not needed, this lining breaks down and is shed, and the blood vessels running through it rupture as it goes. What leaves the body is that lining together with the blood from those vessels.
The ovary does not wait for the shedding to finish. From the very day the flow begins, it starts forming a new egg inside a new follicle.
Endometrium A lining made thick and crowded with blood vessels → an embryo arriving from the oviduct can sink into it and be held → the vessels lie close enough to supply it at once.
13.3.2 The Follicular Phase
This phase runs from about day 5 to day 12, and it is named for what is happening in the ovary: the follicle formed at the start of the cycle is growing.
A growing follicle is not merely a container. As it enlarges, the amount of oestrogens the ovary produces steadily increases, and these hormones travel in the blood to the uterus.
There they act on the uterus and cause its lining to become thicker and to develop more blood vessels. The uterus is being made ready in advance — ready to receive a fertilised egg, to let it become fixed in the lining, and to support its development afterwards. Notice the order of events. The preparation is finished before there is any egg to receive.
13.3.3 The Ovulatory Phase
On about the 13th or 14th day the ripe follicle ruptures. The egg is released from the ovary and begins its journey down the oviduct.
Neither the ripening nor the release happens on its own. Two hormones produced by the pituitary gland — a small endocrine gland lying at the base of the brain — control them. They are the follicle stimulating hormone, or FSH, and the luteinizing hormone, or LH, and together they promote the maturation and the release of the ovum.
So the timing of the cycle is not settled in the ovary alone. The pituitary sets it, the ovary carries it out, and the uterus responds to what the ovary secretes.
Ovulation And Menstruation Ovulation is the release of an egg from the ovary, at about day 14. Menstruation is the shedding of the uterine lining, beginning on day 1. Different events, in different organs, about two weeks apart.
13.3.4 The Luteal Phase
The last phase lasts from day 15 to day 28, and it takes its name from the structure that now dominates the ovary. Once the ovum has been released, the emptied follicle turns into the corpus luteum, which produces hormones of its own.
Under their influence the lining of the uterus thickens still further. By the end of this phase it is at its thickest and richest, which is exactly the state a fertilised egg arriving from the oviduct would need it to be in.
What happens next depends entirely on one question: was the egg fertilised?
If it was, the corpus luteum continues to release progesterone, and the uterus is held in a state fit for the embryo to become fixed in it. The cycle stops here, and pregnancy begins.
If it was not, the ovum disintegrates and the corpus luteum stops producing progesterone. Without progesterone the thickened endometrium cannot be maintained. On the 28th day it begins to shed once more, losing blood that escapes through the cervix and the vagina — and that shedding is the menstrual phase of the next cycle.
Fig. 13.3-1 — Reading The Two Bands Together Take the horizontal scale first. It runs in days, from day 1 at the left to day 28 at the right, so moving to the right means moving forward in time within one cycle. Now read the middle band, the uterus wall. Its height at any point is the thickness of the lining on that day. The height is least at the far left, around days 1 to 5, because the lining has just been shed. From about day 5 the profile climbs. It goes on climbing past day 14 and reaches its greatest height in the days before day 28. So the uterus spends roughly three weeks building its lining and a few days losing it. Now look directly above, at the ovary band, and read the two bands at the same day. Where the profile begins to climb, at about day 5, the ovary shows a growing follicle. Where the profile is highest, the ovary shows the corpus luteum. That vertical alignment is the whole point of the figure. The lining does not thicken by itself. It thickens while the follicle is growing and while the corpus luteum is active, because it is responding to what those structures secrete. Finally, look at the sharp fall at the extreme right. It comes immediately after the corpus luteum is drawn as regressing. When the source of the hormone breaks down, the lining that depended on that hormone breaks down too.
Draw a straight line and mark it off in days from 1 to 28. Shade in the four phases using the day ranges given above, put a single arrow at the day the egg is released, and mark the days on which the lining of the uterus is thickest.
13.3.5 The Fate Of The Released Egg
One egg leaves the ovary in each cycle, and there are only two things that can become of it. Both routes are set out in FC 13.4.
If a sperm is present in the oviduct, the egg is fertilised. It begins dividing at once and becomes an early embryo, and it travels on down into the uterus, where it becomes fixed in the prepared lining. That fixing is the start of pregnancy.
If no sperm is present, the egg degenerates in the oviduct, and part of it may be absorbed. What remains passes on into the uterus and leaves the body in the menstrual discharge, along with the lining that had been prepared for it.
Everything in this section has turned on the presence or absence of a sperm at the right moment. How a sperm comes to be there, and what happens when it reaches the egg, is the subject of the next section.
13.4 Fertilisation
Everything so far has kept the two gametes apart. The male system makes sperms and delivers them; the female system makes an egg and moves it into the oviduct. Fertilisation is the moment those two separate lines meet.
Fertilisation The fusion of the male gamete (sperm) with the female gamete (ovum) to form a zygote.
Notice what the definition insists on. It is not the arrival of a sperm at the egg, and not the entry of a sperm into the egg. It is the fusion of the two, and until the two nuclei have joined, fertilisation has not happened.
13.4.1 The Journey Of The Sperms
During copulation, also called mating or coitus, the sperms are released into the vagina, near the cervix of the uterus. From there they must travel the length of the female passage on their own.
They swim actively, driven by the lashing of their tails, and pass through the cervix into the cavity of the uterus. Their progress is slow — approximately 1.5 mm per minute. From the uterus they climb upward to reach the oviducts, and if an ovum happens to be lying in an oviduct, it is fertilised there.
Almost none of them arrive. Out of the hundreds of millions released into the vagina, very few are able to climb as far as the upper parts of the oviducts. The rest die along the way and are absorbed. The enormous number released is the answer to an enormous rate of loss.
One Sperm From Vagina To Egg 1. The sperm is deposited in the vagina, near the cervix, in a pool of semen along with hundreds of millions of others. 2. The alkaline secretion of the prostate gland in that semen neutralises the acid of the vagina, so the sperm survives its first minutes. 3. Its tail lashes and drives it forward at about 1.5 mm per minute — it has no other means of moving. 4. It passes through the narrow cervix into the cavity of the uterus. Most of its companions do not. 5. It crosses the uterus and enters the opening of one oviduct, climbing against the direction in which the egg is being carried down. 6. High in the oviduct it meets the ovum, and here it is one of only a few hundred left out of the hundreds of millions that set out.
Timing decides everything. A few days soon after ovulation — around the 12th or 13th day after the onset of menstruation — are the most favourable for conception, that is, for becoming pregnant. Sperms may stay alive in the female genital passage for a few days, but their power to fertilise lasts only one or two days.
13.4.2 Structure Of The Sperm
A sperm is a single cell stripped down to the few things it needs. It carries no food store and almost no cytoplasm, because it must travel light and travel far. Its three regions are shown in Fig. 13.4-1.
The Head
At the very tip of the head lies the acrosome. It is a cap, and it works as a chemical drill: it secretes an enzyme, hyaluronidase, which dissolves the wall of the ovum and so lets the sperm enter. Without it a sperm could reach the egg and still be shut out.
Behind the acrosome is the nucleus, which carries the sperm’s genetic material. A human body cell contains 46 chromosomes, arranged as 23 pairs. A gamete carries only one of each pair, so a sperm nucleus carries 23 chromosomes — either \(22 + X\) or \(22 + Y\).
At fertilisation this nucleus combines with the nucleus of the egg, which always carries \(22 + X\). The two half-sets add up, and the normal number of 23 pairs is restored in the new individual.
The Middle Piece
The middle piece contains the sperm’s mitochondria. These provide the energy, as ATP, for all the activity of the sperm, and above all for the swimming that carries it to the egg.
The Tail
The tail is built around a fine axial filament running down its length. It makes lashing movements from side to side, and these drive the sperm forward.
Middle Piece Packed with mitochondria → a steady supply of ATP → the tail can go on lashing for hours → the sperm can cover the whole length of the female passage.
13.4.3 Structure Of The Ovum
The ovum is the opposite kind of cell. It is far larger than a sperm, it cannot move itself at all, and it carries a store of cytoplasm for the embryo’s earliest divisions. Its nucleus carries 23 chromosomes, always \(22 + X\).
It does not lie bare. Around the egg is a thick, clear, non-cellular coat, the zona pellucida, and outside that lies a loose layer of follicle cells clinging to its surface, the corona radiata — the crown of cells it brings with it from the follicle. Between the zona pellucida and the surface of the egg itself is a narrow gap, the perivitelline space. These coats are drawn in Fig. 13.4-2.
Now the work of the acrosome makes sense. A sperm arriving at the ovum does not meet the egg; it meets a crown of cells and a thick coat, and it must get through both.
Zona Pellucida And Corona Radiata The zona pellucida is the thick clear coat lying directly around the egg. The corona radiata is the loose layer of follicle cells lying outside the zona pellucida.
Table 13.3 sets the two gametes side by side.
Table 13.3 — The Sperm Compared With The Ovum
| Where produced |
Seminiferous tubules of the testis |
Follicles in the peripheral part of the ovary |
| Size |
Very small |
Much larger than a sperm |
| Movement |
Motile, swims by lashing its tail |
Non-motile, carried along by cilia and by muscular contractions |
| Parts |
Head, middle piece and tail |
A single rounded cell with coats around it |
| Cytoplasm |
Very little |
Plentiful, a food store for the early embryo |
| Chromosomes carried |
\(22 + X\) or \(22 + Y\) |
\(22 + X\) only |
| Number released |
Hundreds of millions in a single ejaculation |
Normally one in each menstrual cycle |
13.4.4 Fusion Of The Two Gametes
Several sperms may reach the ovum and surround it, as Fig. 13.4-2 shows. Only one of them enters.
The one that enters leaves its tail behind outside. Only the head, which contains the nucleus, penetrates the egg — so the genetic material goes in and the swimming machinery does not.
The instant that happens, the egg answers. A chemical barrier forms at its surface and the entry of every other sperm is prevented. This is not a matter of chance or of speed alone; the egg actively shuts the door behind the first arrival.
Inside, the sperm nucleus moves towards the egg nucleus and the two fuse. The other sperms outside die and disintegrate. The single cell now formed, carrying 23 pairs of chromosomes drawn from two parents, is the zygote. The stages are drawn in Fig. 13.4-3.
Q2. Millions of sperms set out towards one egg. Why does only one of them ever get in?
A. Because as soon as the first sperm penetrates the ovum, a chemical barrier forms at the egg’s surface and blocks the entry of all the rest. The others are shut out, and they die and disintegrate outside.
A sperm swims at about 1.5 mm per minute, and an oviduct is about 12 cm long. Work out roughly how many minutes a sperm would need to swim the length of one oviduct. Compare your answer with the one or two days for which a sperm can fertilise.
The zygote is a single cell, and it is lying high in the oviduct. It is a long way from the uterus, where it must end up. What it does on that journey is the subject of the next section.
13.5 Implantation And Pregnancy
A zygote is one cell lying high in the oviduct. A baby is a body of many millions of cells lying in the uterus. Between those two states come a few days of dividing and a journey of a few centimetres, and this section follows both.
13.5.1 Cleavage And The Blastocyst
The zygote does not wait until it arrives anywhere. Soon after it is formed it begins to divide, and this early series of divisions is called cleavage.
One cell becomes 2, the 2 become 4, then 8, then 16, and so on. The divisions follow one another quickly and the cells do not grow much in between, so the whole cluster stays about the size of the original egg. What is increasing is the number of cells, not the bulk.
At the solid-ball stage the cluster is called the morula — a solid spherical mass of cells produced by repeated division of the zygote. All the while it is being carried down the oviduct towards the uterus.
Then the ball hollows out. By the time it reaches the uterus it has become a blastocyst: a hollow sphere of cells with a single cellular layer, the trophoblast, forming its wall, and an inner cell mass projecting into the cavity from one side. Fig. 13.5-1 follows the whole journey from ovary to uterus, and Fig. 13.5-2 shows the blastocyst itself.
The division of labour in that hollow ball matters. The inner cell mass will become the embryo. The outer layer will not; it becomes the tissue that fastens the blastocyst to the mother and draws supplies from her.
Morula And Blastocyst The morula is a solid ball of cells. The blastocyst is a hollow ball, with an outer layer and an inner cell mass. The morula comes first, and hollows out to become the blastocyst.
13.5.2 Implantation
The blastocyst arriving in the uterus does not simply rest on the lining. It sinks into it, forming a pit in the endometrium and becoming fixed there, about a week — 5 to 7 days — after ovulation.
This is implantation, and its timing explains a great deal of Section 13.3. The endometrium has been thickening throughout the cycle precisely so that there is something soft and richly supplied with blood for the blastocyst to sink into. Arrive a fortnight earlier and there would be nothing there.
Implantation The process of fixing of the blastocyst into the wall of the uterus, that is, into the endometrium.
Once the blastocyst is fixed, the woman is pregnant. Pregnancy is the state of carrying the unborn young one inside the body, and it begins at implantation, which is why implantation is also called conception.
13.5.3 Stages In The Development Of The Embryo
From here the uterus goes on developing to take care of what is growing inside it, and the new individual passes through a named series of stages. FC 13.5 gives the sequence in order.
The first four of these stages happen in the oviduct; everything from implantation onward happens inside the uterus. What each stage looks like is set out below.
- Embryo, at about 3 weeks — a tiny organism roughly the size of a large pea. It hardly resembles a human being at all. An embryo is a growing egg after fertilisation, up to the time when the main parts of the body and the internal organs have started to take shape.
- Advanced embryo, at about 5 weeks — the heart and the blood vessels have formed and the circulatory system is working, but there are still no limbs.
- Foetus, from about 8 weeks — the limbs have appeared and there is some resemblance to the eventual human being. The name changes when the growing individual begins to look like a baby.
- Infant — born at the end of nearly 40 weeks.
The order tells you something about how a body is built. The heart and the blood vessels are complete weeks before there are any limbs, because nothing else can grow until there is a system to supply it.
Table 13.4 — The Embryo Compared With The Foetus
| Period |
From fertilisation to about 8 weeks |
From about 8 weeks until birth |
| Appearance |
Hardly resembles a human being |
Recognisably human, with limbs |
| Chief event |
The main body parts and internal organs take shape |
The parts already formed grow and mature |
| Size at the start of the stage |
About that of a large pea at 3 weeks |
Much larger, and growing steadily |
13.5.4 The Foetal Membranes And The Amniotic Fluid
An embryo fixed in the wall of the uterus is soft, growing and entirely helpless. In the early stages of gestation the implanted blastocyst develops a set of membranes around itself, which protect it and help supply it with nutrition as it develops. Because they lie outside the embryo’s own body and never become part of it, they are called the extraembryonic membranes.
There are three of them — the amnion, the chorion and the allantois — together with the yolk sac, and they persist right up to the time of birth. The chorion is the outermost of the three, and it is where the chorion meets the wall of the uterus that the placenta forms. Fig. 13.5-3 shows how they are arranged around the growing foetus.
The Amnion And Its Fluid
The amnion is a sac that develops around the embryo, and it appears early — even before the allantois is formed. The space between the amnion and the embryo is filled with a watery liquid, the amniotic fluid, so that the developing individual grows suspended in liquid rather than resting against a wall.
That single arrangement does four separate jobs.
- It protects the embryo from physical damage caused by jerks or mechanical shocks — for example, if the mother falls over.
- It keeps an even pressure all around the embryo, so that no one part is pressed harder than another.
- It allows the foetus some restricted movement, which growing limbs and muscles need.
- It prevents the foetus from sticking to the amnion.
Amniotic Fluid A liquid completely surrounding the embryo → a sudden jolt is spread evenly instead of striking one point → the foetus is cushioned, held at even pressure, and still free to move.
Put a raw egg in a jar, fill the jar to the brim with water and close it. Shake it gently, then empty out the water and shake the jar the same way again. The difference in how the egg is knocked about is the difference the amniotic fluid makes.
The membranes solve the problem of protection. They do not solve the problem of supply — a foetus growing for forty weeks needs food and oxygen continuously, and it must get rid of wastes just as continuously. The structure that manages all of this is the subject of the next section.
13.6 The Placenta
A foetus is a living organism, and it has all the needs of one. It requires food and oxygen continuously. It produces carbon dioxide and nitrogenous wastes continuously, and these must be carried away.
It can do none of this for itself. It does not eat, it does not breathe air, and it has no working kidney or lung to fall back on. Every one of these jobs is done for it by a single organ, the placenta.
Placenta The intimate connection established between the foetal membranes and the wall of the uterus, permitting the diffusion of nourishment from the mother’s blood into that of the growing foetus, and the disposal of wastes from the blood of the foetus into that of the mother.
13.6.1 Structure Of The Placenta
The placenta is a disc-like structure attached to the wall of the uterus. It is joined to the foetus by a cord containing blood vessels, the umbilical cord, so the foetus itself lies free in its fluid, connected to the mother at one point only.
The working part of the placenta is a set of minute finger-like projections called villi, and there are two sets of them. One set is given out by the wall of the uterus; the other set is given out by the chorion of the embryo, which the allantois supplies with blood vessels.
These two sets interlock, the projections of one set fitting into the spaces between the projections of the other. What they never do is open into one another. The blood of the embryo is brought extremely close to the blood of the mother, and the two never mingle. Fig. 13.6-1 shows the arrangement.
Follow the blood on each side. Foetal blood is brought to the placenta by an artery in the umbilical cord, carrying the carbon dioxide and urea the foetus has produced, and it flows through very fine capillaries inside the foetal villi. Lying close against those capillaries, on the other side of a thin wall, are the capillaries and blood sinuses of the mother.
Placental Villi Two sets of fine finger-like projections interlocked → an enormous surface where the two blood streams lie a thin wall apart → substances diffuse across rapidly while the blood itself never mixes.
Umbilical Artery And Umbilical Vein The umbilical artery carries blood away from the foetus, loaded with carbon dioxide and urea. The umbilical vein carries blood to the foetus, loaded with oxygen and food. Here the artery carries the deoxygenated blood.
13.6.2 Functions Of The Placenta
Everything the placenta does, it does by diffusion across that thin barrier. Substances pass from the side where they are in higher concentration to the side where they are lower, and traffic runs in both directions at once. Table 13.5 lists what crosses each way.
Table 13.5 — Materials That Can Pass Through The Placenta
| Mother to foetus, for utilisation |
Oxygen; glucose; amino acids; lipids, fatty acids and glycerol; vitamins; mineral ions such as sodium, potassium, calcium and chlorine; certain drugs; alcohol and nicotine; antibodies; viruses |
| Foetus to mother, for elimination |
Carbon dioxide; urea; other waste products |
Read the table by function rather than by name and the placenta turns out to be doing the work of three organ systems at once. Oxygen in and carbon dioxide out is respiration. Glucose, amino acids, fats, vitamins and mineral ions in is nutrition. Urea and other wastes out is excretion.
One Oxygen Molecule From Mother To Foetus 1. Oxygen carried in the mother’s blood reaches the blood sinuses in the wall of the uterus, right against the placenta. 2. On the other side of the barrier, foetal blood low in oxygen has arrived through the umbilical artery and is flowing through the capillaries inside the foetal villi. 3. Oxygen is more concentrated in the mother’s blood than in the foetal blood, so it diffuses across the thin wall of the villus. 4. Only the dissolved oxygen crosses. The blood on either side stays where it is; nothing mixes. 5. The foetal blood, now oxygenated, collects and travels back along the umbilical vein in the umbilical cord. 6. It reaches the foetus and delivers the oxygen to the tissues that are growing.
Q4. The mother’s blood and the unborn baby’s blood never mix. So how does food and oxygen get from one to the other?
A. Through the interlocking villi of the placenta. They bring the two blood streams within a thin wall of each other, and dissolved substances — oxygen and food one way, carbon dioxide and urea the other — diffuse across that wall while the blood on each side stays separate.
Push the teeth of two combs into each other and hold them up to the light. See how much of each comb now lies close to the other without the two ever joining. That is the arrangement of the two sets of villi in the placenta.
13.6.3 Permeability Of The Placenta
The placenta is not an open window. It is permeable to the respiratory gases, to nutrients and also to antibodies, and this last point matters — antibodies from the mother cross into the foetus and give the newborn some protection against disease from its first day.
It does not allow germs to pass from mother to foetus. There are exceptions: certain viruses can cross the placenta, including HIV if the mother is already living with the infection.
There is a second reason for keeping the two circulations apart, and it has nothing to do with germs. The mother’s blood is at a relatively high pressure. If there were a direct continuity of blood between mother and foetus, that pressure would seriously damage the soft and delicate tissues of the foetus. The placenta lets substances through while holding the pressure back.
Why Medicines Are Checked During Pregnancy Certain drugs, alcohol and nicotine appear in the list of substances that cross the placenta. Anything dissolved in the mother’s blood that is small enough to diffuse can reach the foetus, and the foetus has no way of dealing with it. This is why doctors check carefully which medicines are safe during pregnancy, and why a pregnant woman is advised to avoid alcohol and tobacco.
13.6.4 The Placenta As An Endocrine Gland
The placenta has one more role, quite separate from exchange. It also acts as an endocrine gland — an organ that secretes hormones into the blood.
It produces oestrogens and progesterone, the same two hormones the corpus luteum was releasing earlier. Progesterone keeps the uterus in a state fit to hold the pregnancy, so the supply is taken over by the placenta as the placenta develops.
This has a practical use. Progesterone appears in the urine, and certain tests for pregnancy work by detecting it there.
13.7 Gestation And Parturition
The full term of the development of the embryo inside the uterus is called gestation. In human beings it lasts for about 280 days, which is the nearly 40 weeks named at the end of FC 13.5.
At the end of that term the uterus does what its thick muscular wall was built to do. The muscles contract powerfully, the cervix widens, and the baby is pushed out — head first. The head is the largest part of the body to be delivered, so once it has passed through, the rest follows easily.
Muscular Wall Of The Uterus The thickest sheet of muscle in the body → it can stretch around a full-term foetus for months and then contract powerfully → the baby is pushed out through the cervix and vagina.
The act itself has a name of its own.
Parturition The act of expelling the full-term foetus from the mother’s uterus at the end of gestation.
Gestation And Parturition Gestation is the whole period of development inside the uterus, about 280 days. Parturition is the single event that ends it — the expulsion of the full-term foetus.
After The Baby Is Born
The newborn announces its arrival by letting out a sharp cry. The cry is not merely a sound; it is nature’s way of clearing the baby’s respiratory passage, and from that moment the lungs take over the work the placenta had been doing.
Within a few minutes the blood vessels inside the umbilical cord shrink, so the cord can safely be tied and cut. The small scar it leaves is the navel.
After about 15 minutes the placenta breaks away from the wall of the uterus and is expelled. Because it comes after the baby, it is called the after-birth. The uterus then contracts and gradually returns to its normal size over the next few days. The stages are drawn in Fig. 13.7-1 and set out in order in FC 13.7.
Why 280 Days Gestation is counted from the first day of the last menstrual period, because that is the one date that can be known for certain. Fertilisation happens about two weeks later, so the embryo itself develops for roughly 266 days. Either way the figure works out at about 40 weeks.
13.8 Twins And Multiple Births
One ovulation, one egg, one sperm, one baby — that is the usual course, and everything in this chapter so far has assumed it. Twins arise when one of those steps is doubled, and it makes a great difference which step it was. FC 13.8 shows the two routes.
13.8.1 Fraternal Twins
Sometimes two eggs are released from the ovaries at the same time, and both of them may be fertilised. Two zygotes are formed, they develop side by side, and two individuals are born.
Twins produced in this way from two separate eggs are called fraternal twins. Since the two zygotes were made from different eggs and different sperms, the children may be both boys, or both girls, or one boy and one girl.
In every respect they are like ordinary brothers and sisters. They resemble each other no more and no less than any other pair of children of the same parents. The one thing that is unusual about them is their age: they were born within a few minutes of each other. About 75 per cent of all cases of twins are of this type.
Biology gives them two more names, and each one names the doubled step. They are diovular, produced from two ova, and dizygotic, produced from two zygotes — that is, from two fertilised eggs.
13.8.2 Identical Twins
The second route begins with a single fertilised egg. During the early stages of cell division, that one zygote may split and separate into two parts. Each part then behaves like an independent egg, and each produces one complete individual.
Twins produced this way are identical twins, and two things follow directly from their single origin. They are always of the same sex — either both boys or both girls. And they are very similar to each other in appearance, since both were built from the same genetic material.
Even so, they are not copies in every detail. Fingerprints differ between identical twins, and so do birth marks and even handwriting. Genetic material sets a great deal, but not everything.
Fig. 13.8-1 sets the two kinds of twins beside an ordinary single birth, and Table 13.6 compares them point by point.
Table 13.6 — Identical Twins Compared With Fraternal Twins
| Origin |
One egg fertilised by one sperm |
Two eggs fertilised by two different sperms |
| Number of zygotes |
One, which splits during early cleavage |
Two, formed separately |
| Other names |
Produced from a single egg |
Diovular or dizygotic |
| Sex |
Always the same — both boys or both girls |
Both boys, both girls, or one of each |
| Resemblance |
Very similar, though fingerprints, birth marks and handwriting differ |
No more alike than ordinary brothers and sisters |
| Share of all twins |
The smaller share |
About 75 per cent of all cases |
Q5. Why do some twins look exactly alike, while others look no more alike than any brother and sister?
A. Because they begin differently. Identical twins come from one zygote that split in two, so both carry the same genetic material. Fraternal twins come from two eggs fertilised by two different sperms, so they are simply two children of the same parents who happen to share a birthday.
Press your thumb on an ink pad and then on paper, and ask a friend to do the same beside it. Compare the loops and ridges. Even twins who share the same genetic material produce two different patterns like these.
Conjoined Twins Conjoined twins come from a single egg, like identical twins, but the two parts fail to separate completely during early division. The place and the extent of the join vary a great deal from one pair to another, and in some cases the twins can be separated surgically.
13.8.3 Triplets, Quadruplets And Higher Multiples
Sometimes three, four or even more babies are born from a single pregnancy, and these arise from a variety of combinations of the two routes.
Triplets, three born at a time, may be produced in either of two ways. One single zygote may split into two and one of those parts may split again, giving three. Or two eggs may be involved, one zygote producing a pair of identical twins while the other develops normally into one baby.
Quadruplets, four born at a time, may be produced as two pairs of identical twins from two eggs, or in other combinations of identical and fraternal twins.
The chances that all the children of a multiple birth survive fall as their number rises, since the uterus and the placenta must support more and more. Rare cases of six or seven babies are known, and fewer of them survive.
When Conception Needs Help Some people are unable to conceive, and one common cause is a blockage of the oviducts, so that the sperm and the egg can never meet. In in-vitro fertilisation, a mature ovum is taken from the ovary and fertilised in a shallow dish in the laboratory — in vitro means in glass. The embryo is allowed to develop to a certain stage and is then placed into the uterus, where it implants and grows normally. Babies born this way are often called test-tube babies, though no test tube is involved; the first such birth was in 1978, and many thousands have followed. Where a woman cannot carry a pregnancy herself, a surrogate mother may carry the fertilised egg or embryo in her own womb until the pregnancy is complete.
13.9 Summary
- Reproduction is the formation of new individuals by sexual or asexual means, the new individuals being able to repeat the process in their own turn.
- In asexual reproduction a single parent gives rise to offspring without any sex cells joining; the reproductive unit may be the whole parent body or a fragment of it.
- Amoeba reproduces by binary fission, in which the parent cell divides into two daughter cells and no parent is left over.
- Hydra reproduces by budding, in which a bud grows out of the body, separates and lives independently, while the parent survives.
- Asexual reproduction is rapid and gives many offspring, but all of them carry the parent’s genetic material, so there is little or no variation.
- In sexual reproduction two gametes — the sperm from the male and the egg or ovum from the female — fuse at fertilisation to form a zygote, which develops into the offspring.
- Sexual reproduction is slower and gives fewer offspring, but the mixing of genetic material from two parents produces variation.
- In human beings, reproduction takes place only by the sexual method.
- Secondary sexual characters are the outward differences between the two sexes. They take no direct part in reproduction, but they let members of a species recognise and attract the opposite sex.
- In human beings they include the distribution of hair on the body and face, breasts in females, muscularity and build, skeletal structure, psyche and behaviour, and a deeper voice in males.
- The primary reproductive parts are the gonads — testes in the male, ovaries in the female — which produce the gametes.
- The accessory reproductive parts are the ducts, glands and other structures that carry the gametes, bring them together, and shelter and nourish the developing young.
- The male reproductive system consists of a pair of testes, the sperm ducts, the accessory glands, the penis and the urethra.
- The testes lie outside the body in the scrotum because sperms mature only at a temperature about 2 to 3 °C below that of the body.
- In the embryo the testes lie in the abdomen and descend into the scrotum shortly before birth; failure to descend leads to sterility.
- Each testis is divided into about 250 lobules containing seminiferous tubules, where sperms are produced by spermatogenesis, and interstitial or Leydig cells, which produce testosterone.
- Sperms pass from the seminiferous tubules through a network of tubules and 12 to 14 efferent ducts into the epididymis, where they are stored, mature and become motile.
- From the epididymis the sperm duct, or vas deferens, carries them through the inguinal canal into the abdomen, where it joins the urethra behind the urinary bladder.
- The seminal vesicles provide the medium in which the sperms are transported and become active; the prostate gland provides an alkaline secretion that neutralises the acid of the vagina; the bulbo-urethral glands provide a lubricant.
- Semen is the mixture of sperms with these secretions. A single ejaculation is 2 to 3 ml and contains 200,000,000 to 400,000,000 sperms.
- The penis transfers the sperms into the vagina. It becomes rigid when blood fills its erectile tissue faster than it can drain away.
- The female reproductive system consists of a pair of ovaries, a pair of oviducts or Fallopian tubes, the uterus, the vagina and the vulva.
- A maturing egg lies inside a follicle, which enlarges and fills with fluid to become the Graafian follicle. The formation of the mature ovum is oogenesis.
- Ovulation is the rupture of the ripe Graafian follicle and the release of the egg. What remains of the follicle becomes the corpus luteum, which secretes oestrogen and progesterone.
- The fimbriae of the oviducal funnel pick up the released egg, and cilia and peristalsis carry it down the oviduct, about 12 cm long, towards the uterus.
- The uterus is a hollow, pear-shaped muscular organ about 7 cm by 5 cm, whose narrow lower neck is the cervix.
- The vagina, 10 to 15 cm long, receives the penis during copulation and is the passage through which the baby is born.
- The vulva is the external genitalia. It carries the separate openings of the urethra and the vagina, the labia minora and labia majora, and the clitoris.
- Puberty is the period during which the immature reproductive system matures and becomes capable of reproducing — at about 10 to 14 years in girls and 11 to 15 years in boys.
- Oestrogens bring about the female secondary sexual characteristics and testosterone the male ones, while progesterone prepares the uterus to receive an embryo.
- The menstrual cycle lasts about 28 days and is counted from the first day of one flow to the first day of the next. Menarche at about 13 years and menopause at about 45 years mark its beginning and its end.
- In the menstrual phase, the first 3 to 5 days, the thickened endometrium is shed along with blood.
- In the follicular phase, days 5 to 12, a follicle grows and the oestrogen it releases thickens the endometrium and enriches it with blood vessels.
- In the ovulatory phase, about days 13 to 14, follicle stimulating hormone and luteinising hormone from the pituitary bring about the maturation and release of the ovum.
- In the luteal phase, days 15 to 28, the corpus luteum secretes progesterone, which keeps the lining thick and ready.
- If the egg is fertilised the corpus luteum continues and pregnancy begins. If it is not, the corpus luteum regresses, progesterone falls, and the lining is shed as the next menstrual flow.
- Fertilisation is the fusion of the sperm with the ovum to form a zygote, and it takes place in the oviduct.
- Sperms deposited near the cervix swim at about 1.5 mm per minute up through the uterus into the oviduct, and very few of them complete the journey.
- A sperm has a head bearing the acrosome and the nucleus, a middle piece packed with mitochondria, and a tail containing an axial filament.
- The acrosome releases hyaluronidase, which dissolves the covering of the ovum. The sperm nucleus carries \(22 + X\) or \(22 + Y\) chromosomes and the ovum \(22 + X\), so their fusion restores the normal 23 pairs.
- The ovum is the largest cell of the body and is surrounded by the perivitelline space, the zona pellucida and the corona radiata.
- Only one sperm enters the egg. Its tail is left outside, and a chemical barrier then prevents the entry of any other sperm.
- The zygote undergoes cleavage as it travels down the oviduct, forming first a solid morula and then a hollow blastocyst.
- Implantation is the fixing of the blastocyst into the endometrium, about 5 to 7 days after ovulation, and it produces the state of pregnancy.
- The embryo has a heart and blood vessels by five weeks. From about the end of the eighth week the limbs have appeared and it is called a foetus, and birth follows at nearly 40 weeks.
- The extraembryonic membranes are the amnion, the chorion and the allantois. The amniotic fluid between the amnion and the embryo protects it from shocks, keeps pressure even all round it, allows it restricted movement, and prevents it sticking to the amnion.
- The placenta is a disc-like structure attached to the uterine wall and joined to the foetus by the umbilical cord. Its foetal and maternal villi interlock but never open into each other, so the two bloods come close but never mix.
- Oxygen, glucose, amino acids, lipids, vitamins, mineral ions and antibodies pass from mother to foetus, and so do certain drugs, alcohol, nicotine and viruses. Carbon dioxide, urea and other wastes pass from foetus to mother.
- The placenta keeps germs out, holds back the high pressure of the mother’s blood, and also acts as an endocrine gland producing oestrogens and progesterone.
- Gestation, the full term of development inside the uterus, lasts about 280 days.
- Parturition is the expulsion of the full-term foetus by powerful contractions of the uterine muscles, head first, after which the placenta is discharged as the afterbirth.
- Fraternal twins, also called diovular or dizygotic, come from two eggs fertilised by two sperms and may be of either sex. Identical twins, also called uniovular or monozygotic, come from one zygote that split in two and are always of the same sex.
- Triplets, quadruplets and higher multiples arise from combinations of these two routes, and the chance of all the children surviving falls as their number rises.
13.10 Key Terms
- Accessory reproductive parts — the ducts, glands and other structures that carry the gametes, bring them together and shelter the developing young, but do not make gametes.
- Acrosome — the cap at the tip of a sperm’s head, which releases the enzyme that dissolves the covering of the ovum.
- Afterbirth — the placenta, discharged from the uterus a short while after the baby is born.
- Allantois — one of the three extraembryonic membranes that develop around the embryo after implantation.
- Amnion — the sac that develops around the embryo and encloses the amniotic fluid.
- Amniotic fluid — the fluid between the amnion and the embryo, which cushions the embryo and keeps the pressure around it even.
- Asexual reproduction — reproduction by a single parent, without any sex cells joining.
- Axial filament — the thread running through the tail of a sperm, along which the lashing movement passes.
- Binary fission — asexual reproduction in which a parent cell divides into two daughter cells, leaving no parent behind.
- Blastocyst — the hollow ball of cells, with an outer layer and an inner cell mass, that reaches the uterus and implants there.
- Budding — asexual reproduction in which an outgrowth of the parent body separates and grows into a new individual.
- Bulbo-urethral glands — a pair of small glands opening into the urethra, whose secretion acts as a lubricant; also called Cowper’s glands.
- Cervix — the narrow lower neck of the uterus, opening into the vagina.
- Chorion — the outermost of the extraembryonic membranes, which gives out the foetal villi of the placenta.
- Cleavage — the early series of divisions by which the zygote becomes a many-celled ball without growing in size.
- Clitoris — a small, highly sensitive erectile structure at the uppermost angle of the vulva.
- Conception — the establishing of a pregnancy.
- Copulation — the act by which semen is released into the vagina; also called mating or coitus.
- Corona radiata — the layer of follicle cells surrounding the released ovum.
- Corpus luteum — the yellow hormone-secreting mass that the emptied follicle becomes after ovulation.
- Efferent ducts — the 12 to 14 fine ducts that carry sperms from the network of tubules in the testis to the epididymis.
- Egg — the female gamete, together with any nutritive and protective layers added around it.
- Embryo — the developing young one from fertilisation until the main body parts and internal organs have begun to take shape.
- Endometrium — the innermost lining of the uterus, which thickens each cycle and is shed at menstruation.
- Epididymis — the long coiled tube capping the testis, in which sperms are stored and become motile.
- Fertilisation — the fusion of the sperm with the ovum to form a zygote.
- Fimbriae — the finger-like ciliated processes of the oviducal funnel, which pick up the released egg.
- Foetus — the developing young one from about the end of the eighth week, once the limbs have appeared, until birth.
- Follicle — the sac of cells in the ovary that holds and nourishes a maturing egg.
- Follicle stimulating hormone (FSH) — a pituitary hormone that promotes the maturation of the follicle.
- Fraternal twins — twins developed from two eggs fertilised by two sperms; also called diovular or dizygotic twins.
- Gametes — the sex cells, the sperm in the male and the ovum in the female, each carrying half the instructions for a new individual.
- Gestation — the full term of development of the young one inside the uterus, about 280 days in human beings.
- Gonads — the primary reproductive organs that produce the gametes: the testes and the ovaries.
- Graafian follicle — the enlarged, fluid-filled follicle that bulges at the surface of the ovary just before ovulation.
- Hyaluronidase — the enzyme released by the acrosome, which dissolves the covering of the ovum.
- Hymen — the thin membrane that partially closes the opening of the vagina in young females.
- Identical twins — twins developed from a single zygote that split in two; also called uniovular or monozygotic twins.
- Implantation — the fixing of the blastocyst into the endometrium of the uterus.
- Inguinal canal — the passage in the abdominal wall through which the testis descends into the scrotum, carrying its duct and vessels.
- Interstitial cells — the packing cells lying between the coils of the seminiferous tubules, which produce testosterone; also called Leydig cells.
- Labia majora — the larger, hair-bearing outer folds at the sides of the vulva.
- Labia minora — the smaller inner folds at the sides of the vulva, lying within the labia majora.
- Luteinising hormone (LH) — a pituitary hormone that promotes the release of the ovum from the ripe follicle.
- Menarche — the first menstrual flow in a young female, at about 13 years of age.
- Menopause — the permanent stopping of menstruation, at about 45 years of age.
- Menstrual cycle — the repeating cycle of about 28 days in the ovary and uterus during the reproductive period.
- Menstruation — the monthly discharge of blood and the broken-down uterine lining through the vagina, when the egg has not been fertilised.
- Middle piece — the region of the sperm between head and tail, packed with mitochondria that supply energy for swimming.
- Morula — the solid ball of cells formed by repeated division of the zygote.
- Oestrogen — the female sex hormone that thickens the uterine lining and brings about the female secondary sexual characteristics.
- Oogenesis — the process by which the ova-producing cells of the ovary give rise to a mature ovum.
- Ovaries — the female gonads, which produce the ova and the female sex hormones.
- Oviducal funnel — the funnel-shaped mouth of the oviduct, lying close to the ovary.
- Oviducts — the pair of tubes that carry the egg from the ovary to the uterus; also called Fallopian tubes or uterine tubes.
- Ovulation — the rupture of the ripe Graafian follicle and the release of the egg from the ovary.
- Ovum — the female gamete itself, the egg cell without any added layers.
- Parturition — the act of expelling the full-term foetus from the uterus at the end of gestation.
- Penis — the male organ that transfers the sperms into the vagina and also passes out urine.
- Perivitelline space — the clear space between the membrane of the ovum and the zona pellucida.
- Placenta — the disc-like connection between the foetal membranes and the uterine wall, through which the foetus is supplied and its wastes removed.
- Pregnancy — the state of carrying the unborn young one inside the body.
- Primary reproductive parts — the gonads, the only organs that produce gametes.
- Progesterone — the hormone that prepares and maintains the uterine lining for an embryo.
- Prostate gland — the bilobed gland around the urethra whose alkaline secretion neutralises the acid of the vagina.
- Puberty — the period during which the immature reproductive system matures and becomes capable of reproducing.
- Quadruplets — four babies born from a single pregnancy.
- Reproduction — the formation of new individuals by sexual or asexual means, which can repeat the process in their own turn.
- Scrotum — the thin-walled sac of skin outside the body in which the testes are held.
- Secondary sexual characters — the outward differences between the sexes that take no direct part in reproduction.
- Semen — the milky fluid made of sperms together with the secretions of the male accessory glands.
- Seminal vesicles — a pair of glands whose secretion serves as the medium in which sperms are transported and become active.
- Seminiferous tubules — the coiled tubes inside the lobules of the testis in which sperms are produced.
- Sexual reproduction — reproduction in which two gametes, usually from two parents, fuse to form a zygote.
- Sperm — the male gamete, a motile cell with a head, a middle piece and a tail.
- Sperm duct — the tube that carries sperms from the epididymis to the urethra; also called the vas deferens.
- Spermatogenesis — the process of formation of sperms in the seminiferous tubules.
- Sterility — the inability to produce sperms.
- Testes — the male gonads, which produce sperms and testosterone.
- Testosterone — the male sex hormone, produced by the interstitial cells of the testis.
- Triplets — three babies born from a single pregnancy.
- Trophoblast — the single outer layer of cells forming the wall of the blastocyst.
- Umbilical cord — the cord containing blood vessels that connects the foetus to the placenta.
- Urethra — the tube that carries urine out of the bladder, and in the male carries semen as well.
- Uterus — the hollow, pear-shaped muscular organ in which the embryo grows and develops.
- Vagina — the muscular tube from the uterus to the exterior, which receives the penis and through which the baby is born.
- Villi — the minute finger-like projections, one set from the uterine wall and one from the chorion, that interlock to form the placenta.
- Vulva — the external female genitalia, containing the separate openings of the urethra and the vagina.
- Zona pellucida — the thick transparent coat immediately outside the ovum.
- Zygote — the single cell formed by the fusion of a sperm and an ovum.
13.11 Points To Ponder
- Asexual reproduction is faster, needs no partner and produces far more offspring, yet no large animal relies on it. What does that suggest about which matters more to a species over a long stretch of time — the number of offspring, or the differences between them?
- The testes are slung outside the body, where they are easily injured, because sperms will not mature at body temperature. Why might a body be built to move the organ to a cooler place rather than to produce sperms that work at the temperature it already has?
- Hundreds of millions of sperms are released and exactly one is used. Is this simply waste, or is there something the large number achieves that a small number could not? Consider the length of the journey and the timing of ovulation before you answer.
- The endometrium is built up and shed again in every cycle, at a real cost in tissue and blood. Why might the uterus rebuild it each month instead of keeping it permanently thick and ready?
- The placenta allows antibodies through but keeps most germs out, and it allows alcohol and certain drugs through as well. What does that combination tell you about how such a barrier is likely to sort what passes — by whether a substance is useful, or by something else about the substance itself?
- Identical twins begin with one set of instructions divided in two, yet their fingerprints, birth marks and handwriting are not the same. If the instructions are identical, what else must be shaping these features?
13.12 Objective Questions
Select the most appropriate option in each case.
- Sperms are stored, and become capable of movement, in the: (1 mark)
- seminiferous tubule
- epididymis
- seminal vesicle
- inguinal canal
- The nucleus of a human sperm carries: (1 mark)
- \(22 + X\) chromosomes only
- \(22 + Y\) chromosomes only
- \(22 + X\) or \(22 + Y\) chromosomes
- 23 pairs of chromosomes
- The layer of follicle cells that surrounds a released ovum is the: (1 mark)
- zona pellucida
- trophoblast
- corona radiata
- perivitelline space
- The developmental stage that becomes fixed in the endometrium is the: (1 mark)
- zygote
- morula
- blastocyst
- foetus
- The membrane that encloses the fluid in which the foetus floats is the: (1 mark)
- chorion
- allantois
- amnion
- endometrium
- In a 28-day menstrual cycle, the luteal phase extends over: (1 mark)
- days 1 to 5
- days 5 to 12
- days 13 to 14
- days 15 to 28
For the questions that follow, choose the answer from the codes given below. (a) Both A and R are true, and R is the correct explanation of A. (b) Both A and R are true, but R is not the correct explanation of A. (c) A is true but R is false. (d) A is false but R is true.
Assertion (A): The testes descend from the abdomen into the scrotum shortly before birth. Reason (R): Sperms do not mature at the temperature of the interior of the body. (1 mark)
Assertion (A): Once one sperm has entered the ovum, no other sperm can enter it. Reason (R): The tail of the entering sperm is left behind outside the ovum. (1 mark)
Assertion (A): The blood of the mother and the blood of the foetus mix freely inside the placenta. Reason (R): Oxygen and glucose reach the foetus from the mother’s blood. (1 mark)
Match each structure in Column I with the function in Column II that belongs to it. (5 marks)
| a. Seminiferous tubule |
i. Secretes progesterone after ovulation |
| b. Corpus luteum |
ii. Dissolves the covering of the ovum |
| c. Acrosome |
iii. Produces sperms |
| d. Fimbriae |
iv. Neutralises the acid of the vagina |
| e. Prostate gland |
v. Picks up the egg released from the ovary |
- Fill in the blanks with a suitable word. (4 marks)
- The process of formation of sperms in the testis is called ______.
- The narrow lower neck of the uterus is the ______.
- The solid ball of cells formed by repeated division of the zygote is the ______.
- The permanent stopping of menstruation at about 45 years of age is called ______.
- State whether each statement is true or false. (4 marks)
- Fertilisation in human beings takes place in the uterus.
- The corpus luteum is formed from the remains of the follicle after ovulation.
- The umbilical vein carries deoxygenated blood from the foetus to the placenta.
- Identical twins are always of the same sex.
- Name the following. (5 marks)
- The enzyme released by the acrosome of a sperm.
- The canal in the abdominal wall through which a testis descends.
- The two hormones of the pituitary gland that bring about the maturation and release of the ovum.
- The single layer of cells forming the wall of the blastocyst.
- The structure discharged from the uterus a short while after the baby is born.
- Choose the odd one out from each group and give a reason for your choice. (4 marks)
- Seminal vesicle, prostate gland, bulbo-urethral gland, epididymis
- Amnion, chorion, allantois, endometrium
- Morula, blastocyst, zygote, corpus luteum
- Binary fission, budding, fertilisation, vegetative propagation
13.13 Check Your Concepts
Define ovulation and implantation. (2 marks)
Give a reason for each of the following. (2 marks)
- The prostate gland pours an alkaline secretion into the semen.
- The wall of the vagina is highly elastic.
Name the two hormones secreted by the corpus luteum, and state one function of each. (2 marks)
State the function of the acrosome and of the middle piece of a sperm. (2 marks)
Differentiate between a morula and a blastocyst, stating the basis of difference. (3 marks)
Differentiate between fraternal twins and identical twins, stating the basis of difference. (3 marks)
Explain how the amniotic fluid protects the developing foetus. (3 marks)
Why does the lining of the uterus break down towards the end of a cycle in which the egg has not been fertilised? Explain the hormonal reason. (3 marks)
Trace the path of a sperm from the place where it is produced to the place where it leaves the male body, naming in order every structure through which it passes. (4 marks)
What would happen if both testes remained inside the abdomen throughout life? Explain your answer. (4 marks)
Explain, with reference to the structure of the placenta, how oxygen reaches the foetus although the blood of the mother and the blood of the foetus never mix. (4 marks)
Describe the changes that take place in the ovary and in the lining of the uterus between day 1 and day 28 of a menstrual cycle in which fertilisation does not occur. (5 marks)
Human beings reproduce only by the sexual method, although asexual reproduction is faster and gives more offspring. Explain, with three points of difference between the two methods, what sexual reproduction provides that asexual reproduction cannot. (5 marks)