58 THE ORIGIN OF MAN 



in fishes and amphibians, and is called the allan- 

 tois. This allantoic sac provides the embryo with 

 necessary oxygen, passing this and other gases 

 through the porous shell. Finally the embryo 

 breaks through the shell and begins to use its 

 lungs. A brief description of some of the early 

 reptiles might be helpful. The Cotylosauria 

 were the solid-skulled reptiles and as they were 

 the most primitive of the group, they retained 

 many of the ancestral stegocephalian character- 

 istics, but there was at least one pair of temporal 

 openings and the teeth were not placed in 

 grooves. The skull was armored with sculptured 

 plates, the pineal eye was large and a skin armor 

 was sometimes present. They (cotylosaurs) 

 were small and all were carnivorous though some 

 fed on plants. The Pelycosaurs were more active 

 with longer tails which were however not used for 

 swimming. They were fiercely carnivorous and 

 in certain forms the teeth were enormous and in 

 others somewhat flattened. They, like the cotylo- 



