464 



MAMMALIA— PHYSIOLOGY 



unite into a few beneath the elongated epididymis which continues to 

 form the vas deferens. These canals transport the spermatozoa to 

 the urethra. By the activity of the ciliated epithelial cells lining the 

 efferent ducts, the then immotile spermatozoa are slowly passed 

 toward the epididymis. The penis, by means of which sperms are 

 transferred, receives the urethra. The prostate glands surround 

 the urethra dorsolaterally a short distance from the urinary bladder. 

 They secrete a fluid which serves as a medium for the spermatozoa. 

 The spermatozoan has a long flagellum, and its activity is affected 



by changes (CO2, acidity) in 

 the seminal fluid, just as the 

 beat of the cilia may also be 

 affected. Cowper's glands are 

 situated posterior to the pro- 

 state glands and secrete a vis- 

 cid fluid which passes into the 

 urethra by a duct from each 

 gland. 



Propagation Rate in Mam- 

 mals. — In the lower verte- 

 brates, such as the cod fish, it 

 is stated that of six million eggs 

 spawned, less than a third are 

 fertilized. In birds and mam- 

 mals, a limited number of eggs 

 are matured at one time. 

 Sometimes a single ovum de- 

 velops at the expense of others. 

 Marshall quotes Aral to the effect that the ovary of the rat at birth 

 contains 35,100 ova, which are reduced by degeneration to 11,000 

 after 23 days, and to 6,000 by the 63d day. Smith and Engle, by 

 the implantation of anterior hypophyseal substance in infantile 

 female mice, succeeded in inducing the ripening of ova such that 

 one of the animals produced twenty young. (See Hypophysis, 

 p. 448.) 



The Nervous System. — There are two main groups of activities 

 in the vertebrate organism which have determined the general plan 

 of organization of the nervous system. 



Reactions toward the external world consist In the finding and 

 capturing of food, fighting with other animals, preparing nests or 



Fig. 



150. Human embryo at six weeks. 

 (Photo by Newton Miller.) 



