STUDY OF CELL MECHANICS 511 



was followed under high power, the two asters shifted their posi- 

 tions until the spindle lay in the plane shown in figure B. The 

 bending of the rays soon disappeared and the egg divided. 



A short description of the behavior of monaster eggs, produced 

 by shaking, has been given by Boveri ('03), and certain phases 

 of their history has been taken up by the author ('15). From 

 these descriptions, it is clear that while most of the monaster 

 eggs pass through a cycle of changes comparable to the changes 

 going on during normal cleavage, a few shift directly into an 

 amphiaster, a short time after the monaster is formed, and divide. 

 It is from the eggs of this class that spiral asters are derived. 



The first sign of the formation of a spiral aster is found in 

 typical monaster eggs at the period when the controls are in the 

 early 2-cell stage. An egg is shown in figure 1 in which a slight 

 bending of the rays may be seen (to the left in the figurfe). A 

 careful inspection of the figure will show that these few bending 

 rays arise from the edge of the centrosphere, run a trifle forward 

 for perhaps a third of their course, and then bend backwards. 

 In other respects, the egg is a typical monaster with the chromo- 

 somes in this case divided forming a half-hemisphere arovmd the 

 aster. 



In figure 2 an egg is shown where the bending process has be- 

 come ^^ery pronounced. Here we note that the centers of twist- 

 ing are localized at two opposite points on the centrosphere. In 

 these regions, the rays arise from the edge of the sphere, run for- 

 ward for a half of their length, perhaps, and then backwards, that 

 is, opposite to the direction in which the aster seems to be moving. 

 It will be seen, also, that the two centers are bending in the same 

 direction, in this case to the right (clockwise). Those portions 

 of the aster which are not involved in the active movement show 

 the rays bending uniformly backwards. Judging from the posi- 

 tion of the chromosomes, the egg is a monaster which has been 

 cut equatorially.- The whole appearance of the figure is what 

 might be expected were there two areas of movement just out- 



- This does not refer to the pohirity of the egg. In monaster eggs the chromo- 

 somes lie on one side of the aster. The plane of section has cut the egg in such a 

 wav that we see the aster in tlie center and the chromosomes on both sides of it. 



