CHAPTER XVI. 



REPRODUCTION. 



IN all except the lowest forms of life, the continuance of the species is 

 effected by means of certain tissues set apart for this purpose. These 

 form cells which develop into a new animal of the same species, 

 the process constituting reproduction. In most animals these cells are 

 of two kinds, namely, spermatozoa and ova, formed by the reproductive 

 organs of the male and female respectively ; a spermatozoon and ovum 

 fuse to form, a new cell which develops into an animal resembling its 

 parents in its general characters. This, again, is capable of reproducing 

 itself, and the cycle of life is completed. 



THE MALE REPRODUCTIVE ORGANS. 



These consist of the testes, which form spermatozoa, and of accessory 

 organs, namely the vesiculae seminales, the prostate gland, the glands 

 of Cowper, and the penis. Each testis is covered by a strong fibrous 

 capsule, the tunica albuginea, from which trabeculse pass into the gland, 

 dividing it into lobules which contain the seminal tubules. Each 

 tubule is convoluted, and consists of a lining epithelium several layers 

 thick, resting upon a laminated basement membrane. The cells 

 nearest the basement membrane are called spermatogonia ; these 

 divide, giving rise to the spermatocytes, which lie more internally. 

 Within the layer of spermatocytes, and formed from them by division, 

 are the spermatids, which develop into spermatozoa. Lying in the 

 connective tissue between the tubules are groups of polyhedral cells, 

 called interstitial cells. The seminal tubules lead into straight tubules 

 (rete testis) which open into the epididymis ; this is a convoluted tube 

 lined by ciliated cells, and is continued as the vas deferens, a thick 

 muscular tube which opens into the prostatic part of the urethra. The 

 vesiculse seminales are branched sacculated outgrowths from the 

 vas deferens. The prostate gland surrounds the first part of the 



410 



