I. 



2. 



Fig. 42. — Modes of segmentation. 



1, 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 un- 

 equally, e.g. frog ; y.s. larger yolk-laden cells ; 3, ovum, with much yolk \y\ tow-ards lower 

 pole, segments partially and discoidally, forming blastoderm [bl.),e.g. bird : 4. ovum, with 

 much central yolk \y), segments partially and peripherally, e.g. crayfish. 



[Facing p. 438. 



