66 



REPRODUCTION AND LTFE HISTORY. 



FIG. 32. Modes of segmentation. 



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



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



2. Ovum with considerable yolk (y.) at lower pole, segments totally 



but unequally, e.g. frcg ; (y.s.) larger yolk-laden cells. 



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



discoidally, forming blastoderm (/'/.), e.g. bird, most fishes. 



4. Ovum, with central yolk (y.), segments partially and peripherally, 



<-.-. most Arthropods. 



