Fig. 42. — Modes of segmentation. 



I, Ovum, with little yolk, segments wholly and equally into a ball of cells (blastula), e.g. 

 sea-urchin ; 2, ovum, with a considerable quantity of yolk(y), segments wholly but unequally, 

 e.g. frog ; vs. larger yolk-laden cells ; 3, ovum, with much yolk (y) towards lower pole, seg- 

 ments partially and discoidally, forming blastoderm (b!.), e.g. bird ; 4, Ovum, with much 

 ■tentral yolk (y), segments partially and peripherally, e.g. crayfish. 



[Facing p. 422. 



