Reproduction 



249 



to the free carrying out of developmental operations — just as the neces- 

 sity of carrying a heavy burden of supplies may impede the progress 

 of a company of explorers. 



Figure 207 represents the cleavage stages of a frog's egg. The suc- 

 cessive divisions follow the same general order as in Amphioxus. Cleav- 

 ages succeed one another at intervals of about an hour, but the period 

 varies with temperature. The yolk evidently hinders cleavage, espe- 

 cially in the vegetal hemisphere. The second cleavage begins at the 

 animal pole before the first is completed at the vegetal pole. In fact, 

 the third cleavage may begin while both first and second are still incom- 

 plete in the region of the vegetal pole. Further, the inequality in size 



ANIMAL POLE 



VEGETAL POLE 



Fig. 206. Ovarian egg of frog; median 

 section. (After Morgan: "The Develop- 

 ment of the Frog's Egg." By permission 

 of The Macmillan Company, publishers.) 



of cells at animal and vegetal poles is much greater than in Amphioxus, 

 another consequence of the greater yolk-mass. 



After the third cleavage, a cavity appears in the midst of the group 

 of eight cells. As cleavages proceed, this cavity enlarges and the em- 

 bryo, as in Amphioxus, becomes a hollow sphere or blastula (Fig. 

 207E). Its cavity (blastocoele) is excentric, occupying approximately 

 the animal hemisphere only. Its wall is more than one cell thick. The 

 great thickness of the wall of the vegetal hemisphere and the conse- 

 quent excentricity of the blastocoele are obviously due to the yolk. 



In Reptiles and Birds. In eggs whose yolk-mass greatly exceeds 

 that of the amphibian egg, all the protoplasm is segregated into a thin 

 plate, the germ-disk, lying on the surface of the relatively enormous 

 mass of yolk (Fig. 208). In such an egg, obviously, there is no mecha- 

 nism for dividing the yolk. Cleavage is confined to the protoplasm of the 

 germ-disk which, following fertilization of its nucleus, splits up rapidly 



