REPRODUCTION 



6i 



times that of an egg of Amphioxus. The greater part of the increased 

 bulk is yolk. The egg (Fig. 38) is strongly polarized with reference to 

 the distribution of the yolk in the protoplasm. From the animal pole 

 where yolk is at a minimum the quantity increases toward the opposite 

 vegetal pole where the maximum occurs. 



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

 development is protoplasm. The developmental behavior of eggs con- 



ANIMAL POLE 



BLASTOCOELE 



Fig. 39. — 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.) 



taining 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 food supplies may impede the progress of a 

 company of explorers. Figure 39 represents the cleavage stages of a 

 frog's egg. The successive divisions follow the same general order as in 

 Amphioxus. Cleavages succeed one another at intervals of about an hour, 

 but the period varies with temperature. The yolk, however, 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, 



