38 



ZOOLOGY 



Fig. 13. Cleavage and gastrulation as affected by yolk (not drawn to scale). The vertical 

 rows At B, C, and D represent different classes of ova. At 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 crowd- 

 ing the protoplasm to one side (a.p.) ; D, ovum with dense yolk collected at centre. The numerals 

 (x-4) indicate stages in cleavage and gastrulation: x, ova; 2, 4-8 celled .stages of segmentation; 

 3, blastomeres, blastula stage; 4, gastrula stage, it, archenteron; a.p.t active pole; bl, blastoderi^; 

 bp., blastopore; ec, ectoderm; en, entoderm; ma, macromeres; mi, micromeres; p.p., passive pole; 

 , s.c, segmentation cavity; y, yolk; y.c, yolk nuclei. 



Questions on the figures. — 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 archenteron? 



