STUDY OF CELL MECHANICS 449 



Monaster eggs may: 



1. Divide early without passing through a period of protoplasmic move- 



ment. Frequently spiral asters are found in such eggs. 



2. Pass through division cycle and: — 



(a) form amphiasters, divide and develop normally. 



(b) form tetrasters or triasters. 



(c) form multiple asters. 



(d) form monasters. 



In the following account the description of the living egg will 

 first be given and then the results of the study of sectioned 

 material. 



Changes in living egg 



The monocentric condition is a normal phase in the develop- 

 ment of an amphiaster, (Boveri '00, p. 155). Consequently, 

 monaster eggs cannot be certainly distinguished until the 

 majority of the controls are in some late phase of the first divi- 

 sion cycle. By this time the single aster will have reached 

 considerable size, and is seen approximately in the middle of 

 the egg with radiations running in all directions. The size 

 of the aster is variable. 



As the control eggs divide, the monaster eggs show little 

 change except a slow increase in the size of the clear centro- 

 sphere. As a few of the controls are entering the early phases 

 of the second cleavage, some of the monaster eggs undergo 

 changes which lead to the formation of an amphiaster, and such 

 eggs divide. The vast majority of eggs, however, show no 

 change until about half the controls are in the 4-cell stage. 

 The single aster, which by this time is very large, begins to 

 undergo a striking change. It slowly becomes flattened on 

 one side, then concave, and finally saucer-shaped. As this is 

 going on it moves through the egg's substance until it lies well 

 against one side of the egg (fig. 1). The convex side of the 

 astter goes forward, during the movement and the fibers on the 

 concave side seem to reach nearly to the opposite side of the 

 egg. There is a great amount of variability in the amount of 

 the flattening, and of the retreat of the aster, as a glance at 

 text figures B, C and D will show. 



