196 



GENERAL PHYSIOLOGY 



unequal in size and their contents differ ; the larger one contains 

 the chief mass of the passive yolk, while the smaller one consists 

 principally of active protoplasm. In this way differences arise 

 which have an important bearing upon the subsequent divisions, 

 and become constantly greater. In the third case, that oi gemmation, 

 only a very small portion of the egg-cell becomes divided off ; this 



Pia. 75. — Formation of the polar bodies in the starfish ; sp, nuclear spindle ; rfcl, first polar body ; 

 rk^, second polar body ; tk, egg-nucleus. 



occurs especially during the maturation of the egg in the forma- 

 tion of the so-called polar bodies or direction-corpuscles, where 

 the process occurs twice in succession (Fig. 75). 



In partial division the groove that separates the two daughter- 

 halves extends not through the whole cell, but through a part 



A B C 



Pig. 76.— Discoidal cleavage of the egg of a cephalopod. (After Watase.) 



only, so that in subsequent divisions the daughter-halves remain 

 united on their under side by a common protoplasmic mass 

 (Fig. 76). This form is termed discoidal cleavage. 



In multiple division, no division whatever of the protoplasm 

 appears at first, but the nuclei alone multiply in the egg- cell ; 

 later, however, they wander to the surface and there surround 



