ONTOGENESIS 



201 



FIG. 81. Cleavage and gastrulation (not drawn to scale). The vertical rows 



A, B, C, and D represent different classes of ova. A, an ovum with little yolk; 



B, one with considerable yolk collected at the lower pole (p.p) ; C, one with a 

 large amount of dense yolk crowding the protoplasm to one side (a.p); D, 

 ovum with dense yolk collected at center. The numerals (1-4) indicate stages 

 in cleavage and gastrulation: 1, ova; 2, 4-8 celled stages of segmentation; 3, 

 blastospheres, blastula stage; 4, gastrula stage, a, Archenteron; a.p, active 

 pole; bl, blastoderm; bp, ec, ectoderm; en, entoderm; ma, macrospheres; mi, mi- 

 crospheres; p.p, passive pole; s.c, segmentation cavity; y, yolk; y.c, yolk cells. 

 (Galloway.) 



