66 
REPRODUCTION AND LIFE HISTORY. 
Fic. 31.—Modes of Segmentation. 
x. Ovum, with little yolk, segments totally and equally into a 
blastosphere, ¢.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 (42), e.g. bird, most fishes. 
4, Ovum, with central yolk, (y.) segments partially and peripherally, 
e.g. most Arthropods 
