332 



VERTEBRATE ZOOLOGY 



scrotum. The penis of the male mammal is homologous with the 

 clitoris of the female and is a structure quite unique among verte- 

 brates. 



m 



~\ r H 



jrrr 



m 



FIG. 172. Brain of dog. A, dorsal, B, ^ventral; C, lateral aspect. B. ol, olfac- 

 tory bulb; Cr. ce, crura cerebri; Fi. p, great horizontal fissure; HH, HH', lateral 

 lobes of cerebellum; Hyp, hyophysis; Med, spinal cord; NH, medulla oblongata; 

 Po, pons Varolii; VH, cerebral hemispheres; Wu, middle lobe (vermis) of cerebel- 

 lum; I-XII, cranial or cerebral nerves. (From Wiedersheim.) 



THE ORIGIN OF MAMMALS 



It has long been held that the mammals are descended from the 

 reptiles, a theory based on the fact that the monotremes, primitive 

 egg-laying mammals, have many distinctly reptilian characters. If 

 mammals descended from any other vertebrate class they must have 



