IX 



MOLLUSCA 



327 



At the next cleavage the first quartette of micromeres are given 

 off. These consist entirely of the white material, though some of this 

 still remains in the macromeres. The polar lobe is again constricted 

 off from D, but it is much smaller than before and the constricting 

 furrow does not extend so deeply. When the cleavage is complete 

 the polar lobe again fuses with ID. 



Before the next cleavage occurs the white material derived from 

 the animal pole, part of which was left in each macromere, increases 



Fig. 256. — Stages in the cleavage of the egg of Dentalium. (After Wilson.) 



A, completion of the first cleavage. B, beginning of tlie second cleavage, seen from the side. C, the 

 second cleavage in its most intense period, seen obliquely from above and the side. %^, the first polar 

 lobe ; ^2j the second polar lobe ; p.&, polar bodies. 



in amount, moves over to the right side of each cell, and extends 

 somewhat down the side. In ID this also occurs, but in this cell the 

 white material from the animal pole is joined by the white material 

 from the vegetative pole, which moves over and fuses with it. Of the 

 second quartette of micromeres 2d is formed first, and it is composed 

 of the white material derived from both poles, whereas 2a, 2b, and 2c, 

 which are formed soon afterwards and likewise consist of white 

 material, have received only white material which was originally at 

 the animal pole of the egg. At the same time the first-quartette 

 cells divide into the trochoblasts (Iq^) and the upper cells, the latter 



