THE RISE OF THE MAMMALIA 111 



in the case of Birds, be supplied with food col- 

 lected by the care of the parents. The egg of the 

 Reptile or Bird is relatively very large, and its size 

 is due to the great amount of yolk which it contains. 

 This yolk is the food material on which the embryo 

 subsists during its development ; it supplies the 

 growing embryo with the material from which it 



Fig. 23. Longitudinal section through a Bird or Reptile embryo in 

 its egg-shell, am. amniotic cavity; em. body of embryo; g. gut 

 of embryo communicating by the umbilicus with the large yolk- 

 sac ys. 



may build up all the various organs of its body. 

 The embryo itself, as it develops, floats, so to speak, 

 upon the yolk, from which it becomes folded off, 

 but to which it remains attached at one point, the 

 umbilicus (Fig. 23). The umbilicus attaches the 

 yolk-sac to the growing embryo, and it is in reality 

 a hollow stalk by which the yolk-sac is in com- 

 munication with the hind part of the alimentary 



