REPRODUCTION IN MAN 1221 



that the blood trickles only slowly from the arterioles into the spaces, 

 whence it escapes readily by means of the veins. If the muscle fibres be 

 relaxed, so that blood can escape readily 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 spermatogonia, 

 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 membrane 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 formed by the testis are mixed not only with 

 the fluid secreted by the lining 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 perpetua- 

 tion 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. 



