The science of life : fully illustrated in tone and line and including many diagrams

WEARING OUT OF MACHINE AND ITS REPRODUCTION

The lowliest and simplest animals do not show this division into two sexes. When we are studying them in a later chapter we shall find quite different methods of reproduction. But in the mammals the creation of a new life necessarily involves the encounter of male and female bodies, and the consequent union of male and female gametes.

The male gametes, or spermatozoa, are curiously constructed. They are very unlike most of the cells we have hitherto considered. Each spermatozoon consists of a rounded head—this is the part that fertilizes the ovum —and a long, lashing tail, by means of which it can propel itself in quest of the waiting female gamete. The whole thing is about one five-hundredth part of an inch long.

The male organs are of three kinds. There is a pair of roundish bodies, the testes, where these spermatozoa are manufactured, a series of passages along which the spermatozoa pass to be injected into the female system, and in which they wait until they are wanted, and a series of accessory glands whose secretions mix with the sperm as it is discharged.

A testis consists, in the main, of a close mass of coiling tubes, lined by ceaselessly proliferating cells. Some of these cells are converted into spermatozoa, the nucleus becoming the sperm-head, part of the cytoplasm becoming the sperm-tail, and the rest of the cell dropping off and disintegrating. Others simply multiply, giving rise to new sperm- # + forming cells. In between the coiling tubes there are cells of another kind, which have nothing to do with sperm-formation, but have quite another function in the body.

From the testis the spermatozoa enter a fine, tortuous tube, the epididymis, which, although between twenty and thirty feet long in the human case, is curled up into a mass smaller than the testis itself. In this canal the spermatozoa are stored until required, when they are forced out of it by muscular movements of its wall. From the epididymis the spermatozoa pass along a tube to the urethra, a passage that guides them out of the body and which, by a curious economy, they share with the urine. The last part of the urethra projects and the walls about it are spongy in structure. When spermatozoa are discharged these walls are stiffened by distension with blood in order

to facilitate insertion into the recipient female passage.

A number of accessory glands open into the urethra—the prostate, the so-called seminal vesicles, and Cowper's glands. These glands manufacture fluids, which are discharged at the same time as the spermatozoa. The fluids serve to nourish the spermatozoa, and they seem to spur them to activity, for the spermatozoa lie passive in the epididymis, and do not begin to lash and swim until they are mixed with the secretions of the accessory glands.

Nature is curiously lavish with spermatozoa. At each human sexual act as many as two hundred million spermatozoa may be put

Bladder

Ureter

Seminal Vesicle

Prostate Gland

» Epididymis

Testis”

Fig. 60. Diagram of the Male Organs of Reproduction in a higher mammal as seen from the left side.

forth, of which only one (in rare cases two or three) may achieve its object and fertilize an ovum. ;

The female gamete or egg (ovum) is very much larger than the male. The human egg cannot compare in size with that of a bird, but it is nevertheless just visible to the naked eye. It is a spherical cell, about a one hundred and twenty-fifth part of an inch in diameter, and surrounded by a transparent pellicle ; besides the indispensable nucleus, its protoplasm includes a number of granules of yolk, the fuel that is needed to provide the energy for the earliest stages of development. The female organs (Fig. 61) include the ovaries, where the ova are made, female equivalents to the testes, and a series

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