EMBRYOLOGY 



219 



Cleavage in Telolecithal Eggs. — The extent to which, in the presence 

 of mucii yolk, cleavage departs from the regular method described for 

 homolecithal eggs, appears to depend on how much yolk is present, and 

 how sharply it is separated from the protoplasm of the egg. In the lam- 

 prey, which may be described as "mildly" telolecithal, the egg is divided 



Fig. 171. — Blastula of the lamprey, two stages, in section. The cells are very unequal 

 in size, and the blastoccele is eccentric in position, b, blastocoele; 7na, macromeres; 7ni, 

 micromeres. 



much as in Synapta, except that the third cleavage plane is far from equa- 

 torial (see Fig. 170). The upper quartet of cells in the eight-cell stage 

 is much smaller than the lower. Also, in the lamprey, the irregularities 

 of division occur much earlier and are much more obvious. A blastula 



Fig. 172. — Cleavage of the telolecithal Fig. 173. — Blastula of telolecithal egg, 



egg of the gar-pike, Lepidosteus ossexis, about diagrammatically represented in vertical 

 5 hrs. after fertilization. The yolk-laden section, b, blastoccele; c, cleavage cells, 

 vegetative half of the egg remains undivided. 

 (After Eycleshymer, courtesy ofGustm Fischer.) 



is formed, but the blastocoele is eccentric in position (see Fig. 171). The 

 wall of cells is much thicker and is composed of much larger cells at the 

 vegetative pole than at the animal pole. Cleavage in the frog's egg is 

 very similar to that of the lamprey. 



In the eggs of fishes the yolk and protoplasm are almost completely 



