66 



REPRODUCTION AND LIFE HISTORY. 



FIG. 31. Modes of Segmentation. 



j. 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 (/.), e-g. bird, most fishes. 



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



e.g. most Arthropods 



