STUDY OF CELL MECHANICS 517 



given by a consideration of several facts. Boveri ('05) was the 

 first to suggest that the cytoplasm in the sea urchin egg undergoes 

 a series of changes — a sort of development — independent of the 

 nuclear elements, and I reached the conclusion, after studying 

 eggs treated with narcotics that ''at the time of fertilization 

 progressive changes, which go on independently of the nucleus 

 and of cleavage, are initiated in the cytoplasm of the eggs and 

 that these changes determine the position of the spindles in the 

 egg and consequently in its blastomeres," p. 299 (1. c). 



In monaster eggs the division mechanism is delayed through 

 the failure of the centrosome to divide, but the cytoplasmic de- 

 velopment is not materially interfered with. This is indicated 

 by the fact that if the aster recovers and divides into an amphi- 

 aster, the spindle tends to take up the same relative position as 

 the normal controls have taken and as a consequence we have the 

 early production of the micromeres as Boveri ('05), and I myself 

 have shown. 



The single aster, during its early history takes up a position at 

 or near the center of the egg. The egg cytoplasm, after fertiliza- 

 tion, begins its development which normally, of course, goes hand 

 in hand with certain changes in the nucleus and of the division 

 mechanism. By the production of monasters we are able to 

 separate, for a time, the division mechanism and the cytoplasmic 

 development. When now the division mechanism is released, 

 as through the division of the centriole or the completion of the 

 monaster cycle, the new spindle tends to take up the same posi- 

 tion which the control eggs have. Our observations show that 

 the spiral asters arise from monaster eggs which have recovered 

 very early from the effects of the shaking, when the controls are 

 in the 2- and 4-cell stages. From this we should expect that the 

 young spindle will attempt to take up the position which the 

 controls possess, that is, either in the plane of the first or of the 

 second cleavage. This expectation is fulfilled by the single living 

 egg which was observed. The young spindle, resulting from the 

 division of a monaster, lay at an angle with the pigment band, at 

 a time when the controls were in and 2- and 4-cell stages. Boveri 

 ('01) showed that the first two divisions in Strongylocentrotus 



