REPRODUCTION IN MAN 1273 



relaxed, so that blood can pass rapidly into and distend the spaces, the 

 tissue swells and becomes harder, causing ' erection ' of the organ. 



In the immature testis, i. e. from birth up to puberty, the seminal tubules 

 are filled with cells with large nuclei. Some of these are the sperm atogonia, 

 the mother cells of the future spermatozoa, while the others form the cells 

 of Sertoli, whose function it is to act as nurse cells to the developing sper- 

 matozoa. The actual formation of spermatozoa begins at puberty, when 

 the spermatogonia divide many times to form the spermatocytes, which 

 in their turn undergo heterotype mitosis to form the spermatids, as already 

 described. By a modification of the latter the fully formed spermatozoa 

 are formed. These, when mature, pass by the tubules of the testis and of 

 the epididymis into the vas deferens, whence they make their way into the 

 seminal vesicles. Their movement is probably facilitated by the cells lining 

 the tubule of the epididymis as well as by the secretion of the lining mem- 

 brane of the seminal vesicles. It has been noted that the spermatozoa are 

 practically motionless while in the seminiferous tubules of the testis, but 

 become actively motile in the vas deferens, or when mixed with prostatic 

 secretion. It is difficult to understand how the spermatozoa are conveyed 

 through the resistance which must be offered by the huge length of the 

 tubule of the epididymis, unless their onward motion is facilitated by the 

 cilia-like structures attached to some of the cells lining this tubule. The 

 formation of the spermatozoa is continuous, though the rate at which this 

 occurs is variable and regulated by the sexual activity of the individual. In 

 the fully formed semen the spermatozoa originating in the testis are mixed, 

 not only with the fluid secreted by the fining membrane of the epididymis 

 and of the seminal vesicle, but also with the mucous secretions of the prostatic 

 glands and of Cowper's glands. Nevertheless it contains spermatozoa in 

 enormous numbers, the semen emitted at a single act of coitus containing 

 as many as 226,000,000 spermatozoa. Though the vast majority of these 

 are probably capable of fertilising an ovum, this act is carried out by only 

 one a fact characteristic of the prodigality of nature when dealing with 

 the perpetuation of the type. 



THE FEMALE REPRODUCTIVE ORGANS 



The essential organ of reproduction in the female is the ovary, the seat 

 of production of the ova. The accessory organs include the oviducts or 

 Fallopian tubes, the uterus, in which the fertilised ovum is retained during 

 the first nine months of its development, and the vagina, which is especially 

 adapted for the reception of the male organ in the act of impregnation. 



Among the accessory organs we may also reckon the mammary glands, 

 which undergo a special development during pregnancy, and serve for 

 the nourishment of the young individual during the first period of 

 extra-uterine life. 



OVULATION. At birth the ovary consists of a stroma of spindle-shaped 

 cells, and is covered by a layer of cubical epithelium (the germ epithelium) 

 continuous with the endothelium fining the general peritoneal cavity. 



