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. 



1 lJ-i,U,;!^^^^ 



J^l'p 111 r„ 17 17,Y 



Fig. 172.— Brain of dos- .1, dnrsal, ]i, ventral; (', lateral aspect. P>. ul, olfac- 

 tory bulb; Or. ce, crura cerebri; PH. p, great horizontal fissure; ////, ////', lateral 

 lobes of cerebellum; Hyp, hyophysis; hfal, spinal cord; Nil, medulla oblongata; 

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

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



The Okigin of Mammals 



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



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



egg-laying mammals, have many distincth' reptilian characters. If 



mammals descended from any other ^•ertcbrate class they nuist have 



