CLEAVAGE OF THE OVUM 



77 



V • ♦ • «• * •* • * * • -V 



Fig. 39. — Modes of Segmentation. 



1. Ovum, with little yolk, segments totally and equally into a 



blastosphere, e.g. Hydra, sponge, sea-urchin. 



2. Ovum, with a considerable amount of yolk (y.) at lower pole, 



segments totally but unequally, e.g. 'frog ; {y.s.) larger yolk- 

 laden cells. 



3. Ovum, with much yolk (y.) at lower pole, segments partially and 



discoidally, forming blastoderm [bl.], e.g. bird, most fishes. 

 Ovum, with central yolk (y.), segments partially and peripher- 

 ally, e.^. most Arthropods. 



