SEXUAL reproduction involves the joining or fusing together of two cells. One of these reproductive cells comes from a malee animal and the other comes from a female. The reproductive cells are called gametes. The fusing of two gametes is called fertilization and the resulting, composite cell is called a zygote. The most important aspect of fertilization is the fusion of the nuclei of the male and female gametes, because the factors which determine the characteristics of the individual that grows from the zygote are in these nuclei.

Fertilization, in short, is the fusion of the nuclei of male and female gametes to form a zygote, from which can develop a new individual.

In animals, the male gametes are sperms, which are produced in the reproductive organs called testes. The female gamete is an ovum which is produced in a reproductive organ called an ovary.

Some animals such as earthworms and snails are hermaphrodite, that is, they have both testes and ovaries, but in most animals the sexes are separate.

Internal and external fertilization. In most fish and amphibia fertilization is external. The female lays the eggs first and the male fertilizes them by placing sperms on them afterwards. A behaviour pattern which brings the sexes into proximity usually ensures that sperm is shed near the eggs and so increases the chances of fertilization.

In reptiles and birds, the eggs are fertilized inside the body of the female by the male's passing sperms into the egg ducts. A sperm meets the ovum and fertilizes it before it is laid. Very little development of the egg takes place before laying, however, and the embryo grows in the egg after it has left its mother's body.

In mammals, sperms are placed in the body of the female and the eggs are fertilized internally. They are not laid after fertilization but retained in the female's body while they develop to quite an advanced stage, after which the young are born more or less fully formed, being fed on a secretion of milk from the mammary glands and protected by their parents until they become independent.

Sexual reproduction in man

Female reproductive organs. The female reproductive organs are the o varies, two cream-coloured, oval bodies lying in the lower part of the abdomen below the kidneys. They are attached by a membrane to the uterus and supplied with blood vessels. Close to each ovary is the expanded, funnel-shaped opening of the oviduct, the tube down which the ova pass when they are released from the ovary.

The oviducts are narrow tubes that open into a wider tube, the uterus or womb, lower down in the abdomen. When there iS no embryo developing in it the uterus is only about 80 mm long. It communicates with the outside through a muscular tube, the vagina. The cervix is a ring of muscle closing thee lower end of the uterus where it joins the vagina. There is normally only a very small aperture connecting these two organs at this point. The ure thra, from the bladder, opens into the vulva in front of the vagina.

Male reproductive organs. The two testes lie outside the abdominal cavity in man, in a special sac called the scrotum, consequently the testes remain at a temperature rather below that of the rest of the body, which is favourable to sperm production. The testes consist of a radiating mass of sperm-producing tubes. These tubes meet and join to form ducts leading to the epididymis, a coĆ­led tube about 6m long on the outside of each testis. The epididymis, in turn, leads into a muscular sperm duct. The two sperm ducts, one from each testis, open into the top of the urethra just after it leaves the bladder. A short, coiled tube, the seminal vesicle, branches from each sperm duct just before the latter enters the prostate gland. Surrounding the urethra at this point, is the prostate gland and farther down, Cowper's gland. The urethra conducts, at different times, both urine and sperms, In males the urethra is prolonged into a penis, which consists of connective tissue with numerous small blood spaces in it.

Ovulation. The ovary consists of connective tissues, blood vessels and potential egg cells. It is thought that 70,000 potential egg cells are already present at birth; they are not manufactured by the ovary during the lifetime. Of these 70,000 potential egg cells only about 500 will ever become mature ova (eggs). The ovaries also secrete hormones oestrogens, which control the secondary sexual characters and initiate the thickening of the uterine lining which occurs each month.

Between the ages of about 11 and 16 years the ovaries become active and begin to produce mature eggs. The beginning of this period of life is called puberty. In the ovary some of the ova start to grow; the cells around them divide rapidly and become richly supplied with blood vessels. A fluid filled cavity is eventually produced, and this partly encircles the ovum and its coating of nutritive cells. This region of the ovary is called a Graafian follicle. When the Graafian follicle is ripe it is about the size of a pea and projects from the surface of the ovary.

Finally it bursts and releases the ovum into the funnel of the oviduct, the ciliated cells of which waft it into the tube. At this stage the ovum is a spherical mass of protoplasm about 0-13 mm in diameter, with a central nucleus and some of the follicle cells still adhering to it. An ovum is produced more or less alternately from the two ovaries every four weeks and it may spend about three days travelling down the oviduct to the uterus.

Sperm production. The lining of the tubes making up the testis consists of actively dividing cells which give rise ultimately to sperms. A sperm is a nucleus surrounded by a little cytoplasm which extends into a long tail.

Sperms are quite immobile when first produced; they pass into the epididymis where they are stored. During mating, muscular contractions of the epididymis, sperm duct and accessory muscles force the accumulated sperms through the urethra. Here, secretions of fluids from the prostate gland and seminal vesicles add nutrients and enzymes, diluting the sperms and stimulating them into action, when lashing movements of their tails propel them along.

Fertilization. Fertilization occurs internally when the sperms meet the ovum as it passes down the oviduct. They are introduced into the female through the penis which is placed in the vagina. To facilitate this action the penis becomes erect.

largely as a result of blood flowing into the blood spaces more rapidly than it escapes, so increasing the turgidity of the tissues round the urethra. The stimulation of the sensory organs in the penis sets off a reflex action which results in the accumulated sperms, together with the secretions of the prostate and Cowper's glands, being ejaculated into the vagina. The action is called copulation and may result in fertilization. The sperm deposited in the vagina swim through the cervix into the uterus and up the oviduct. If an ovum is present in the oviduct, one of the sperms will eventually collide with it and the head of the sperm sticks to the ovum. The sperm's nucleus passes into the cytoplasm and fuses with the female nucleus there.

Although a single ejaculation may contain two or three hundred million sperms, only one will actually fertilize the Ovum, though the others may assist in the fertilization as a follicle cells result of an enzyme they produce which helps to disperse the naining follicle cells adhering to the surface of the ovum (not confirmed in man so far). In some mammals such as the rabbit, several ova are released from the ovary at the same time and will be fertilized by the corresponding number of sperms.

In man, the released ovum is thought to live for 12-24 hours, while the sperm might live in the female reproductive organs for 2-3 days. Thus there is only a relatively short period of 3-4 days each month when fertilization can occur.

The cells of the follicle from which the ovum has been released continue to divide and grow, forming a solid body, the corpus huteum. If fertilization does not occur, the corpus luteumn degenerates and gives way to ordinary ovary tissue. If fertilization does occur, the corpus luteum enlarges further and produces ahormone, progesterone, which stimulates the further thickening and increased blood supply of the uterine lining.

Pregnancy and development. The fertilized egg undergoes rapid cell division as it passes down the oviduct and into the uterus where it adheres to and sinks into the uterine lining. Finger-like processes, villi, grow from the embryo into the uterine lining and absorb nourishment. The region bearing the villi does not form part of the cmbryo but develops into a special organ called the placenta which supplies the embryo with both food and oxygen.

The uterus, which at first has a volume of only 2 to 5 cm extends with the growth of the embryo to 50O0 to 7000 cm enlarging the abdomen and displacing the organs in it to some extent. The uterine lining, under the influence of ocstrone and progesterone, develops a rich supply of blood vessels and the walls become increasingly muscular.

The cells of the embryo divide repeatedly to form tissues. The tissues swell, roll, extend, etc., and so form organs of the body The embryo's heart and circulatory system are formed quite carly, after about one month.

Although the embryo depends on the mother's blood for its food and oxygen its circulatory system is never directly connected with the maternal blood vessels. If this did occur, the adult's blood pressure would burst the delicate capillaries forming in the embryo, and many substances in the mother's circulation would be poisonous to the embryo.

The placenta. The placenta becomes a large disc of tissue adhering closely to the uterine lining.

From the placenta villi protrude into the uterine lining which has thickened and developed a rich blood supply. The mem- branes separating the maternal and embryonic blood vessels are very thin; hence dissolved substances can pass across in both directions. Dissolved oxygen, glucose, amino acids and salts in the mother's blood pass from the uterine vessels into those of the embryo, while carbon dioxide and nitrogenous waste from the embryo pass across in the opposite direction. The membrane exerts some selective influence over the substances that pass into the placenta and so prevents harmful material from reaching the embryo. The capillaries in the placenta are connected to an artery and vein which run in the umbilical cord from the embryo's abdomen to the placenta.

The embryo is surrounded by a water sac or amnion which protects it from damage and prevents unequal pressures from acting on it. After about five month's growth the embryo moves its limbs quite vigorously inside the water sac and uterus. From fertilization to birth takes about nine months in humans. This length of time is called the gestation period.

Birth. A few weeks before birth the embryo comes to lie head downwards in the uterus with its head just above the cervix

When the birth starts, the uterus begins to contract rhythmically. This is the onset of what is called "labour. These rhythmic contractions become stronger and more frequent. The opening of the cervix gradually dilates enough to let the child's head pass through, and the uterine contractions are reinforced by muscular contractions of the abdomen. The water sac breaks at some stage in labour and the fluid escapes through the vagina.

Finally, the muscular contractions of the uterus and abdomen expel the child head first through the dilated cervix and vagina. The umbilical cord which still connects the child to the to placenta is tied and cut. Later, the placenta breaks away from the uterus and is expelled separately as afterbirth". 

The sudden fall in temperature experienced by the newly born baby stimulates it to take its first breath, usually accompanied by crying. In a few days the remains of the umbilical cord attached to the baby's abdomen shrivel and fall away, leaving a scar in the abdominal wall, called the navel. The average birth weight of babies is 3kg.

Shortly after its birth the baby starts to suckle. During pregnancy the mammary glands of the breast have developed and are stimulated to secrete milk by the first sucklings.

Parental care. All mammals suckle their young and protect them in various ways until they are old enough to move about efficiently and obtain their own food. Most mammals prepare a nest in which to bear their young, and in this way the babies are protected from animals of prey and from temperature changes. The nest reduces the chances of their wandering and getting lost and prevents them from injuring themselves. The young mammals are often born without fur but the presence of the mother's body prevents them from losing heat. The food at first is entirely milk, suckled from the mother. The milk contains nearly all the food, vitamins and salts that the young need for their energy requirements and tissue building but there is no iron present for the manufacture of haemoglobin. All the iron needed for the first weeks or months is stored in the body of the embryo during the period of gestation in the uterus.

The parent's milk supply increases as the young animals grow larger and is supplemented by solid food. In carnivorous mammals the prey is brought back to the nest site and torn into pieces small enough for the young to swallow. Often the parents are more aggressive when they have young and react violently to intruders. When the young animals are old enough to obtain their own food and escape from predators, they leave the nest site and disperse. In humans, the period of dependence on parents is lengthy.

TwinsIDENTICAL TWINS. Sometimes a fertilized ovum divides into two parts at an early stage of cell division and each part develops separately into a normal embryo. Such "one egg" twins are the same sex and are identical in nearly every physical respect, although differences in position and blood supply while in the uterus may cause them to differ initially in weight and vigour.

FRATERNAL TWINS. If two ova are released from the ovary and fertilized simultaneously, twins will result. These twins may be different sexes and are not necessarily any more alike than other brothers and sisters of the same family.

Secondary sexual characters. In addition to producing gametes, the ovaries and testes make chemicals called hormones. At puberty these hormones are released into the blood stream, and as they circulate round the body they give rise to physical and mental changes which we associate  with masculinity or femininity.

In males, the voice becomes deeper, hair begins to grow on the face, in the armpits and in the region of the external genitalia, and the body becomes more muscular. In females, the hormones cause the breasts to grow and the hip girdle to enlarge. These features are called secondary sexual characters.

Menstruation. Of the 500 or so ova produced in the life of a Woman, not more than 12 are likely to be fertilized and form embryos. Nevertheless, at the time of release of each ovum the lining of the uterus becomes thicker with additional layers of cells into which the ovum will sink if fertilized. The blood supply is increased at the same time. If the ovum is not fertilized, however, the new uterine lining disintegrates and the unwanted cells, a certain amount of blood, and mucus are lost through the cervix and vagina. This menstruation, as it is called, occurs 12-14 days after the egg is released, once in about four weeks.

Birth control. When people wish to limit the size of their families they usually make use of one or other form of birth control. This may involve either a contraceptive practice or restricting copulation to a period in which fertilization is unlikely to occur, i.e. from 5 to 10 or 18 to 28 days after the onset of menstruation. This is not a very reliable method because there is considerable individual variation in the time of ovulation, the time interval between menstrual periods and the regularity of their occurrence.

The principal methods of contraception are as follows:

 (a) The sheath or diaphragm. A thin rubber sheath worn on the penis, or a small rubber diaphragm inserted in the vagina prevents the sperms from reaching the cervix 

(b) Intru-uterine loop. A small plastic strip bent into a loop or coil is inserted and retained in the uterus. Whether it interferes with fertilization or implantation is not certain but it is very effective.

(c) The contraceptive pill. The pill contains chemicals which  This period is calculated on the assumption that (a) human sperms can organs, and (b) that an ovum can be terrified up to 24 hours after ovulatio have the same effect on the body as the hormones oestrogen and progesterone. When mixed in suitable proportions, these hormones suppress ovulation and so prevent conception. The pills need to be taken each day for the 21 days between menstrual periods and are almost 100 per cent effective.

World population. With the increasing application of medical knowledge, fewer people are dying from infectious diseases.

The birth rate, however, has not fallen off in proportion, with the result that the world population is doubling every 50 years or less. In the developing countries with limited natural resources, this leads to shortages of food and living space. In the industrialized countries, the population increase has contributed to the pollution of the environment as a result of waste disposal and intensive methods of food production.

There is clearly a physical limit to the number of people who can live on the Earth, though authorities may differ in their estimates of this number. Therefore it seems essential, at the very least, to educate people (a) into accepting the need to limit their families and (b) in methods of achieving this. The alternative to voluntary population control is control by famine and disaster.