AS is well known reproduction is effected by the growth of a cell which is formed by the joining together of two cells, the ovum and spermatozoon which come from the female and male respectively. Both male and female, therefore, must produce these cells and must contain a system of tubes by which they may be led to one another, and also a place where the fertilised ovum can safely develop. When animals lived in the water it was comparatively simple, for the ovum thrown out into the surrounding water to be fertilised by the male could not be destroyed by drying, and when fertilised could continue its development unhindered except by the raids of other animals that desired it for food.
In these circumstances, therefore, millions of eggs were laid by each female and these were subsequently fertilised by millions of spermatozoa cast at random over the egg mass. Thus was it ensured that even if many were destroyed by predatory animals, a few would survive. When, however, animals left the water some new method of reproduction had to be found in order to prevent the embryo drying in the air. To accomplish this the ovum is fertilised inside the female by spermatozoa which are introduced by the penis into the passage of the female known as the vagina.
Many of the land animals have avoided the difficulty of drying by covering their fertilised eggs with a shell, the egg being laid and continuing its development outside the mother. In the mammals and man, however, the fertilised egg is retained and is developed inside the mother and is born in a state of comparative maturity. Many animals, too, produce large numbers of fertilised eggs so as to ensure the complete development of at least a few. In man, however, only one egg is produced at a time under normal circumstances, for the environment is such that the developing ovum and subse-
quent infant have relatively few dangers to withstand owing to their careful protection. Such have been the factors that have given rise to the rather complicated arrangements for reproduction in the human body.
THE MALE’S PART IN A COMPLEMENTARY SYSTEM THE essentially male organs which produce the spermatozoa are the testicles. These two organs were originally formed, and in the developing embryo actually lie, inside the abdomen, but in the adult they have become pulled down into a bag which lies between the legs. It is difficult to see the reason for this, for they are obviously less protected in this position and, of course, from a racial point of view they are one of our most important possessions. However, it would appear that in the higher mammals the stimulus of relative cold is required for their proper development at puberty, for if by chance they fail to descend or remain in the warmth of the abdomen or if they are artificially replaced there they fail to develop as they should.
Each testicle contains a large number of enormously long and tortuous tubes closely coiled together and very narrow in cross-section. These tubes produce the spermatozoa, which are minute cells with a head and a very long tail which can be waved vigorously so that locomotion is possible. All the tubes of the testes are gathered together finally into a long duct which passes upwards and enters the abdomen, finally reaching the back of the bladder beliind which a sac grows out of the tube in which the spermatozoa can be stored. Out of this leads a very fine tube which traverses the prostate gland surrounding the urethra into which the duct finally leads. Spermatozoa after passing into the urethra are discharged at the proper time from the end of the penis. The prostate gland secretes a fluid which stimulates the spermatozoa to active movement. This secretion comes in contact with them only when they are about to be discharged, for although it makes them more active it materially shortens their life. In the body of the male, if they are not discharged, they can live more or less indefinitely but when passed into the organs of the female they can live at most only for about ten days. Millions upon millions of spermatozoa are discharged at one act of coitus, but it is characteristic of the prodigality of nature that only one of this huge number is responsible for fertilizing the ovum.
CONCEPTION: A MIRACLE OF NATURE’S EVOLUTION THE essential sex organs of the female are the two ovaries which lie in the lower part of the abdomen. They are small, oval objects about the size of a large almond and consist of large numbers of cell groups, which produce the ova, embedded in a mass of supporting tissue. When an ovum ripens it bursts through the coat of the ovary and comes to lie in the lower part of the great abdominal cavity that surrounds the intestines. It lies in this cavity, however, only for a short while, for in close proximity to the ovary is the open end of a tube down which it will later be carried to the womb.
The open end of this tube has its wall frayed out into processes which more or less surround the ovary, and the inside of the tube is lined by cells which possess hairs that by their motion can produce a stream which sucks the ovum into the tube and carries it down its interior, where, if conditions are satisfactory, it becomes fertilised by a spermatozoon which has worked its way upwards from the vagina. The fertilised (or unfertilised) ovum then passes down the remainder of the tube which leads into the upper part of the womb. If the ovum is fertilised it finds the walls of the womb thickened and specially prepared to receive it and it then starts actively to burrow into the substance of the wall. Here it continues its development until it is fully mature and ready to be born.
The walls of the womb are composed of a thick layer of muscular tissue which is used at the end of pregnancy for driving the infant out of the womb. Normally the womb weighs but a few ounces, but during pregnancy it increases enormously in size and its walls become thicker and more richly supplied with blood vessels for the nourishment of the infant until the womb at full term comes to weigh two to three pounds.
The lower part of the womb leads directly into the vagina where the spermatozoa are lodged by the male in the act of coitus. The spermatozoa, after their deposition in the vagina, make their way by the active motion of their tails into the cavity of the womb and from there into the tube where they meet the ovum and fertilise it. We shall see in the section on the ductless glands how the ovary sends out its two messengers into the blood stream, cestrin and lutein, which prepare the womb for the reception of the ovum. We shall see, too,
how the second hormone, lutein, is produced only after the ovum is ripe and has burst its way out of the ovary and how progestin is produced only for long periods if the ovum is fertilised.
If fertilisation does not occur then the production of progestin ceases after ten days, and with its cessation the lining membrane of the womb breaks down and is cast off with a certain amount of blood and constitutes menstruation.