REPRODUCTION 



41 



Yolk is a non-living, quite inert substance. The active material 

 in development is protoplasm. The developmental behavior of eggs 

 containing much yolk shows quite clearly that the yolk is an impediment 

 to the free carrying out of developmental operations — just as the necessity 

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

 company of explorers. 



Figure 35 represents the cleavage stages of a frog's egg. The successive 

 divisions follow the same general order as in Amphioxus. Cleavages 



ANIMAL POLE 



BLASTOCOELE 



Fig. 35. — Cleavage of the frog's egg. A, first cleavage in process; B, two cells; 

 C, eight cells; D, fourth cleavage complete in animal hemisphere but just beginning in 

 the four cells at the vegetal pole; E, early blastula, median section; F, G, successively- 

 later stages, lateral view. {D, F, G, redrawn from Morgan, "The Development of the 

 Frog's Egg"; E, redrawn from Marshall, "Vertebrate Embryology.") 



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

 with temperature. The yolk evidently hinders cleavage, especially 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 incomplete in the region 

 of the vegetal pole. Further, the inequality in size 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 embryo, 



