104 



ELEMENTARY ZOOLOGY. 



FIG. 20. A diagram illustrating the early steps (cleavage and gastrulation) which 

 follow fertilization (Fig. 19, C). The vertical rows A, B, C, and D represent different 

 kinds of fertilized ova. A, an ovum of 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 proto- 

 plasm to the upper pole (a.p.) ; D, ovum with dense yolk collected at the center. The 

 numerals (1-4) indicate stages in cleavage and gastrulation: i, fertilized ova; 2, the 

 4-8-celled stages of cleavage; 3, blastulas sectioned; 4, gastrula stage, a, archenteron, 

 or primitive digestive tract; a.p., active pole; bl, blastoderm; bp, blastopore; ec, ecto- 

 derm; en, entoderm; p.p., passive pole; s.c., segmentation cavity; y, yolk. 



Questions on the Figure. Why is one pole spoken of as active 

 and the other passive? Which pole gives rise to ectoderm? 

 Which to entoderm? Why does food substance interfere with the 

 cleavage? What is the difference between segmentation cavity and 

 archenteron? Note how the amount and arrangement of the food 

 substance modify the archenteron. 



