STUDY OF CELL MECHANICS 



459 



of the cell. As the chromosome vesicles fuse to form one nu- 

 cleus, the rays of the aster appear to break up, (fig. 25) becoming 

 granular, and gradually they entirely disappear. Traces of 

 the lightly staining centrosphere may be found for some time, 

 however. In only a few cases have I found evidence of the 

 retreat of the aster in this type of egg. Two serial sections of 

 such an egg are given in figures 13 and 15. The projections of 

 the protoplasm formed during the movement are here clearly 

 seen. 



Fig. E 



The eggs with a deeply staining centrosphere seem to undergo 

 a different series of changes. Following the division of the 

 centrioles (fig. 19) the centrosphere becomes elongated and 

 moves toward one side of the egg (fig. 19 and text fig. E 2 ). After 

 this two things may happen. The centrioles may separate 

 completely and form a spindle between them. This process 

 is usually accompanied by the formation of the peculiar spiral 

 asters which I have described in another place ('16). The 

 determining factor as to whether or not the spindle is formed 

 seems to be the division of the chromosomes. If the chromo- 

 somes divide before the centrioles are far apart, then no separa- 

 tion appears to take place, instead nuclear vesicles are formed 

 and these fuse to one nucleus as the astral rays disappear. 



Quickly following on the first division cycle, the second one 

 sets in, and the early appearance of the amphiaster is seen from 



