800 



TEXTBOOK OP ZOOLOGY 



the egg and sperm pronuclei, by means of wliich the chromosomal com- 

 plex of the egg is restored to the diploid condition and the hereditary 

 mechanism is enabled to function. The last period is that of cleavage, 

 during which the egg segments into many cells and the egg substances 





Fig. 413. — Fertilization in the sea urchin, Toxopneustes. S, mature spermato- 

 zoon ; m, transformation of spermatozoon into male pronucleus ; /, female pronu- 

 cleus. (Reprinted by permission from Outline of Comparative Embryology by 

 Richards, published by John Wiley and Sons, Inc., after Wilson.) 



are distributed into the appropriate cells, so that differentiation and 

 subsequent division of labor can take place. The formation of germ 

 layers follows the organization of the Uastomeres into the three sheets 

 of tissue whose further development leads to the formation of the 



