STRUCTURE OF THE VERTEBRATES 349 



forms they assist in maintaining a constant temperature. Fur- 

 ther aid in temperature control would be supplied by the dia- 

 phragm, a structure which is well developed in the Alligator 

 and tj^pical of the mammals. 



The embryological processes are further evidence of the kin- 

 ship between reptiles and mammals. The monotremes lay large 

 yolked eggs with meroblastic development like that of the rep- 

 tile. The young are hatched by the body heat of the mother. 

 But in the marsupials and placentals the egg is microscopic 

 and holoblastic. These eggs are structurally very dissimilar 

 from those of a reptile; and yet, as the egg develops, a yolk 

 sac is formed and the embryo assumes the spread-out condition 

 of the reptile with the archenteron connected with the yolk sac. 

 The heart begins its development from paired vitelline veins, 

 and many other developmental similarities exist which can be 

 accounted for in no other way except that it is a repetition of 

 a reptilian scheme. 



But so far removed from the other mammals does the ancestry 

 of the monotremes appear, that the mammals are thought to 

 have arisen as two separate lines from forms which are called 

 ^'reptiles" rather than mammals. Aside from much other evi- 

 dence, the bony structure makes this more than plausible. The 

 shoulder girdle of the monotreme is more reptilian than that of 

 the higher Therapsida. The monotreme skull and ear ossicles, 

 however, are mammalian. 



The evolution of the face has been outlined. The therapsid 

 reptiles had an enlarged calvarium; a shortened, deepened skull; 

 e^^es which were focussed forward and not outward; and a het- 

 erodont dentition. The face and skull were mammalian with the 

 exception of a few reptilian bones. 



The embryology of the ear ossicles was discussed in the sec- 

 tion on the visceral skeleton. The skull of the Therapsida sup- 

 plies the anatomical evidence. The lower jaw of the primitive 

 reptiles is formed of six bones, the dentary being the most 

 anterior. With the rise of the mammal-like reptiles the dentary 

 becomes the largest, and the other bones are pushed far to the 

 proximal end, or are lost. In the higher therapsid types the 

 articular, angular and surangular form a small group posteri- 

 orlv. The articulation of the jaws remains between the quadrate 



