40 



EVOLUTION AND GENETICS 



successive generations of cells. Such division always occurs in the 

 vertebrates, but its results may be superficially very different 

 according to the amount of food included in the cell. The mini- 

 mum is found in the eggs of the lower chordates and many fishes, 



B 



D 



E 



F 



d 



GUI 



Fig. 19. — Early stages in the development of the egg of a sea urchin. A-F, 

 cleavage and formation of the blastula; G, section of blastula showing the 

 beginning of gastnilation; H-I, early and late gastrula stages, a, ectoderm; 

 b, endoderm; c, blastocoele; d, blastopore, leading into the enteric cavity; 

 e, cells arising from the endoderm, destined to form the mesoderm. (From 

 Woodruff.) 



the maximum in the reptiles and birds in whose eggs the entire yolk 

 is the fertilized ovum. In true mammals, whose young are nour- 

 ished by a placental connection with the mother, such a concen- 

 tration of food to supply the developing embryo is unneces.sary, 

 henc(^ the ovum is small. The stored food or yolk is inert matter 

 which delays the process of subdivision. 



