THE DEVELOPMENT OE A MOLLUSK 37 



still farther separated, the radiations become more distinct, and 

 soon seem to act on the chromosomes as two sets of fibers. The 

 next step is a separation of every chromosome into two parts, 

 which seem to be drawn in opposite directions toward the two 

 centrosomes. These changes are shown in Model 2 and Fig. C. 



In this manner two new nuclei are formed from the pronuclei, 

 each new nucleus being composed of one-half of the chromatin 

 of the male and female pronuclei, and each nucleus having a 

 centrosome. 



At the same time that the division of the pronuclei takes 

 place a corresponding division of the whole egg occurs. The 

 egg elongates (Model 2), a constriction takes place, and finally, 

 coincident with the formation of the two new nuclei, there is 



MODEL 3 



Completion of first cleavage. Two cells. Polar bodies in the furrow be- 

 tween them. Daughter nuclei and centrospheres in each cell. 



a complete separation of the egg into two halves, forming two 

 new cells, each made up of protoplasm and yolk, like the single 

 undivided egg, and each having a nucleus with its centrosome 

 (Model 3). One of these two new cells gives rise to the anterior 

 portion of the embryo, the other to the posterior. 



Beginning with this, the first cleavage, up to the time when 

 the larva is capable of taking in new food, the whole process of 



[13J 



