THE DEVELOPMENT OP THE EMBRYO ITSELF. 131 



terior part contains a bone, while there are two erectile portions 

 independent of each other. During copulation the largest (pos- 



FiG. 136.— Section of parts of three aemmiferous tubnles of the rat (Schater). a, with 

 the spermatozoa least advanced in development; b, more advanced; c, containing 

 fully developed spermatozoa. Between the tubules are seen strands of interstitial 

 cells, with blood-vessels and lymph-spaces. 



terior) erectile region is spasmodically (reflexly) grasped by the 

 sphincter cunni of the female, which is the analogu e of the 

 bulbo-cavernosus, ischio-cavernosus, and deep transverse mus- 

 cle of the perinseum, so that the penis can not be withdrawn 

 till the erection subsides, an advantage, considering that the 

 seminal vesicles are wanting in the dog, as well as Cowper's 

 glands. In the cat tribe there is also an incomplete penial 

 bone. The free portion of the organ is provided with rigid 

 papillae capable of erection during copulation. 



As previously explained, the spermatozoa originate in the 

 seminal tubes, from which they find their way to the seminal 

 vesicles or receptacles for semen till required to be discharged. 

 The spermatozoa as they mature are forced on by fresh addi- 

 tions from behind and by the action of the ciliated cells of the 

 epididymis, together with the wave-like (peristaltic) action of 

 the vas deferens. Discharge of semen during coitus is effected 

 by more vigorous peristaltic action of the vas deferens and the 

 seminal vesicles, followed by a similar rhythmical action of the 

 bulbo-cavernosus and ischio-cavernosus muscles, by which the 

 fluid is forcibly ejaculated. 



