REPRODUCTION IN MAN 1365 



of the epididymis the vas deferens, a tube with thick muscular 

 walls, leads by the abdominal ring to the base of the bladder, 

 where it opens into the beginning' of the urethra in its prostatic part. 

 Just before it joins the urethra each vas deferens presents a diverticu- 

 lum, the seminal vesicle, which lies along, and is attached to, the base 

 of the bladder. The prostate itself, which surrounds the first part of 

 the urethra, is composed of a matrix of unstriated muscular fibres, 

 enclosing numerous branched raceme-tubular glands. From the 

 point of entry of the vasa deferentia to its orifice, the urethra repre- 

 sents a common passage for the urine and for the sexual products the 

 semen. It passes therefore through tissues, forming the penis, which 

 are especially adapted for the purpose of intromission, i.e. the intro- 

 duction of the semen containing the spermatozoa into the female. 

 In the urethra we distinguish the prostatic, the membranous, and 

 the penile portions. Into the beginning of the penile portion, the bulb 

 of the urethra, open the ducts of the two glands of Cowper. In the 

 penis itself the urethra is surrounded with erectile tissue, forming the 

 corpus spongiosum, and lies between the two corpora cavernosa, which 

 consist of the same kind of tissue. The erectile tissue is a spongy mesh- 

 work of elastic and unstriated muscle fibres, enclosing spaces in free 

 communication with the efferent veins of the organ. The arterioles 

 also open into these spaces, but under normal circumstances both 

 the arterioles and the muscle-tissues of the framework are contracted, 

 so 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 spermatozoa. 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 modifica- 

 tion 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 sper- 

 matozoa are practically motionless while in the seminiferous tubules 

 of the testis, but become actively motile in the vas deferens, or when 



