Sexual reproduction involves fertilisation In sexual reproduction, the parent organism produces sex cells or gametes. Eggs and sperms are examples of gametes. Two of these gametes then join together. This is called fertilisation. The new cell which is formed by fertilisation is called a zygote. The zygote divides again and again, and eventually grows into a new organism.
Gametes have half the normal number of chromosomes.
Gametes are different from ordinary cells, because they contain only half as many chromosomes as usual. This is so that when two of them fuse together, the zygote they form will have the correct number of chromosomes. Humans have 46 chromosomes, for example, in each of their body cells. But human egg and sperm cells only have 23 chromosomes each. When an egg and sperm fuse together at fertilisation, the zygote which is formed will therefore have 46 chromosomes, the normal number. The 46 chromosomes in an ordinary human cell are 23 different kinds. There are two of each kind. The two chromosomes of one kind are called homologous chromosomes. A cell which has the full number of chromosomes, with two of each kind, is called a diploid cell. An egg or sperm, though only 23 chromosomes, one of each kind, is called a haploid cell. Gametes are always haploid. When two gametes fuse together, they form a diploid zygote.
Gametes are made by meiosis.
Gametes are made by ordinary body cells dividing. For example, human sperm are made when cells in the testis divide. Because gametes need to have only half as many chromosomes as their parent cell, division by mitosis will not do. When gametes are being made, cells divide in a different way, called meiosis. In flowering plants and animals, meiosis only happens when gametes are being made. Meiosis produces new cells with only half as many chromosomes as the parent cell.
Male gametes move - female ones stay alive.
In many organisms, there are two different kinds of gamete. One kind is quite large, and does not move much. This is called the female gamete. In humans, the female gamete is the egg. The other sort of gamete is smaller, and moves actively in search of the female gamete. This is called the male gamete. In humans the male gamete is the sperm. Often one organism can only produce one kind of gamete. Its sex is either male or female, depending on what kind of gamete it makes. All mammals, for example, are either male or female. Sometimes, though, an organism can produce both sorts of gamete. Earthworms and slugs, for example, can produce both eggs and sperms. An organism which produces both male and female gametes is a hermaphrodite. Many flowering plants are also hermaphrodite.
Sexual reproduction in a mammal. The female reproductive organs.
The female gametes, called eggs or ova, are made in the two ovaries. Leading away from the ovaries are the oviducts, sometimes called Fallopian tubes. They do not connect directly to the ovaries, but have a funnel shaped opening just a short distance away. The two oviducts lead to the womb or uterus. This has very thick walls, made of muscle. It is quite small - only about the size of a clenched fist - but it can stretch a great deal when a woman is pregnant. At the base of the uterus is a narrow opening, guarded by muscles. This is the neck of the womb, or cervix. It leads to the vagina, which opens to the outside.The opening from the bladder, called the urethra, runs in front of the vagina, while the rectum is just behind it. The three tubes open quite separately to the outside.
Gametes are made by meiosis.
Gametes are made by ordinary body cells dividing. For example, human sperm are made when cells in the testis divide. Because gametes need to have only half as many chromosomes as their parent cell, division by mitosis will not do. When gametes are being made, cells divide in a different way, called meiosis. In flowering plants and animals, meiosis only happens when gametes are being made. Meiosis produces new cells with only half as many chromosomes as the parent cell.
Male gametes move - female ones stay alive.
In many organisms, there are two different kinds of gamete. One kind is quite large, and does not move much. This is called the female gamete. In humans, the female gamete is the egg. The other sort of gamete is smaller, and moves actively in search of the female gamete. This is called the male gamete. In humans the male gamete is the sperm. Often one organism can only produce one kind of gamete. Its sex is either male or female, depending on what kind of gamete it makes. All mammals, for example, are either male or female. Sometimes, though, an organism can produce both sorts of gamete. Earthworms and slugs, for example, can produce both eggs and sperms. An organism which produces both male and female gametes is a hermaphrodite. Many flowering plants are also hermaphrodite.
Ovaries make eggs
The eggs are made from cells in the outside layer, or epithelium, of the ovary. Some of these cells move towards the centre of the ovary. A small space, filled with liquid, forms around each one. The space and cell inside it is called a follicle. This has happened inside a girl's ovaries before she is born. At birth, she will already have many thousands of follicles inside her ovaries. When she reaches puberty, some of these follicles will begin to develop. Usually, only one develops at a time. The cell inside the follicle grows bigger, and so does the fluid filled space around it. The follicle moves to the edge of the ovary. It is now called a Graafian follicle. It is little more than 1cm across, and bulges from the outside of the ovary. The cell inside it undergoes meiosis. Only one of the cells which are made becomes an egg. The follicle bursts, and the egg shoots out of the ovary. This is called ovulation. In humans it happens about once a month.
The male reproductive organs.
The male gametes, called spermatozoa or sperm, are made in two testes. These are outside the body, in two sacs of skin called the scrotum. The sperms are carried away from each testis in a tube called the vas deferens. The vasa deferentia(plural of vas deferens)from the testes join up with the urethra just below the bladder. The urethra continues downwards, and opens at the tip of the penis.The urethra can carry both urine and sperms at different times. Where the vasa deferentia joins the urethra, there is a gland called the prostrate gland.This makes a fluid which the sperms swim in. Just behind the prostrate gland are the seminal vesicles, which also secrete fluid.
Testes make sperms
The testes contains thousands of narrow, coiled tubes or tubules. These are where the sperms are made. They develop from cells in the walls of the tubules, which divide by meiosis. Sperms are made continually from puberty onwards. Sperms production is very sensitive to heat. If they get too hot, the cells in the tubules will not develop into sperms. This is why the testes are outside the body, where they are cooler than they would be inside.
Mating introduces sperms into the vagina
After ovulation, the egg is caught in the funnel of the oviduct. The funnel is lined with cilia which beat rhythmically, wafting the egg into the entrance of the oviduct. Very slowly, the egg travels towards the uterus. Cilia lining the oviduct help to sweep it along. Muscles in the wall of the oviduct also help to move it, by peristalsis. If the egg is not fertilised by a sperm within 8-24 hours after ovulation, it will die. By this time, it has only travelled a short way along the oviduct. So a sperm must reach an egg while it is quite near the top of the oviduct if fertilisation is to be successful.
Copulation
When a man is sexually excited, blood is pumped into spaces inside the penis, so that it becomes erect. To bring the sperms as close as possible to the egg, the man's penis is placed inside the vagina of the woman. This is called sexual intercourse or copulation. Sperms are pushed out of the penis into the vagina. This happens when muscles in the walls of the tubes containing the sperms cntract rythmically. The wave of contraction begins in the testes, travels along the vasa deferntia, and into the penis. The sperms are squeezed along, and out of the man's urethra into the woman's vagina. This is called ejaculation. The fluid containg the sperms is called semen. Ejaculation deposits the semen at the top of the vagina, near the cervix.
Fertilisation happens in the oviduct
The sperms are still quite a long way from the egg. They swim, using their tails, up through the cervix, through the uterus, and into the oviduct. Sperms can only swim at a rate of about 4 mm per minute, so it takes quite a while for them to get as far as the oviducts. Many will never get there at all. But one ejaculation deposits about a million sperms in the vagina, so there is a good chance that some of them will reach the egg. One sperm enters the egg. Only the head of the sperm goes in; the tail is left outside. The nucleus of the sperm fuses with the nucleus of the egg. This is fertilisation. As soon as the success sperm enters the egg, the egg membrane becomes impenetrable, so that no other sperm can get in. The unsuccessful sperms will all die.
The embryo implants in the uterus wall.
When the sperm nucleus and the egg nucleus have fused together, they form a zygote. The zygote continues to move slowly down the oviduct. As it goes, it divides by mitosis. After several hours, it has formed a ball of cells. This is called an embryo. The embryo obtains food from the yolk of the egg. It takes several hours for the embryo to reach the uterus, and by this time it is a ball of 16 or 32 cells. The uterus has a thick, spongy lining, and the embryo sinks into it. This is called implantation.
The embryo's life-support system is its placenta
The cells in the embryo, now buried in the soft wall of the uterus, continue to divide. As the embryo grows, a placenta also grows, which connects it to the wall of the uterus. The placenta is a soft and dark red, and has finger-like projections called villi. The villi fit closely into the uterus wall. The placenta is joined to the embryo by the umbilical cord. Inside the cord is an artery and a vein. The artery takes blood from the embryo into the placenta, and the vein returns the blood to the embryo. In the placenta are capillaries filled with the embryo's blood. In the wall of the uterus are large spaces filled with the mother's blood. The embryo's and mother's blood do not mix. They are separated by the wall of the placenta. But they are brought very close together, because the wall of the placenta is very thin. Oxygen and food materials in the mother's blood diffuse across the placenta into the embryo's blood, and are then carried along the umbilical cord to the embryo. Carbon dioxide and waste materials diffuse the other way, and are carried away in the mother's blood. As the embryo grows, the placenta grows too. By the times the embryo is born, the placenta will be a flat disc, about 12 cm in diameter, and 3 cm thick.
normal and abnormal placenta
development of foetus
An amnion protects the embryo
The embryo is surrounded by a strong membrane, called the amnion. Inside the amnion is a liquid called amniotic fluid. This fluid helps to support the embryo, and to protect it.
A baby develops during gestation
No one fully understands how the cells in the ball which embedded itself in the wall of the uterus become arranged to form a baby. The cells gradually divide and grow. By eleven weeks after fertilisation they have become organised into all the different organs. By this stage the embryo is called a foetus. After this, the foetus just grows. It takes nine months before it is ready to be born. This length of time between fertilisation and birth is called the gestation period.
Muscular cntractions cause birth
A few weeks before birth, the foetus usually turns over in the uterus, so that it is lying head downwards. Its head lies just over the opening of the cervix. Birth begins when strong-muscles in the wall of the uterus start to contract. This is called labour. To begin with, the contractions are quite gentle, and only happen about once an hour. Gradually, they become stronger and more frequent. The contractions of the muscles slowly stretch the opening of the cervix. After several hours, the cervix is wide enough for the head of the baby to pass through. Now, the muscles start to push the baby down through the cervix and the vagina. This part of the birth happens quite quickly. The baby is attached to the uterus by the umbilical cord and the placenta. Now that it is in the open air, it can breathe for itself, so the placenta is no longer needed. The placenta falls away from the wall of the uterus, and passes out through the vagina. It is called the afterbirth. The umbilical cord is cut, and clamped just above the point where it joins the baby. This is completely painless, because there are not nerves in the cord. The stump of the cord forms the baby's navel.
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Contributor's Note
I live in Trinidad and Tobago and we follow the british system using the english language contrary to Americans. For example.
british english - labour, american english - labor
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