FROM SIMPLE CELL TO COMPLEX ANIMAL. 



FIG. ii. 



33 



FIG. ii. 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..) ; D, ovum with dense yolk collected 

 at centre. The numerals (1-4) indicate stages in cleavage and gastrulation: i, ova; 

 2, 4-8 celled stages of segmentation; 3, blastospheres, blastula stage; 4, gastrula stage. 

 a, archenteron; a. p., active pole; bl, blastoderm; bp., blastopore; ec, ectoderm; en, ento- 

 derm; ma., macrospheres; mi, microspheres; p.p., passive pole; s.c., segmentation 

 cavity; y, yolk; y.c., yolk cells. 



Questions on the figure. -What constitutes the difference between the 

 active and the passive pole? Judging from the drawings and from your 

 references to texts . does gravity have any influence in determining the 

 position of these? Your evidences? Which pole gives rise to ectoderm? 

 Why does the food substance interfere with segmentation? What is the 

 difference between the segmentation cavity and the archenteron? How 

 does the presence of food substance modify the formation of an archen- 

 teron? 



