STUDY OF CELL MECHANICS 521 



significance and is not due simply to a rearrangement of materials, 

 as Conklin found, for example, in Crepidula. It may be possible 

 that it is a reminiscence of a former cleavage which has now 

 been omitted from the ontogeny of Nephelis. This explanation 

 does not seem improbable. However this may be, the main 

 point for us is that since the rays run forward and then back- 

 wards, the cause of the bending can not he in the asters themselves 

 but must be in the protoplasm outside of these division areas. 



This short review of a few of the papers dealing with spiral 

 asters (I have made no effort to give a complete bibliography) is 

 sufficient to show that the conclusion reached, after a study of 

 the sea urchin egg, is valid for a number of other forms. 



There are several points of general interest which should be 

 mentioned here, as our observations throw some light upon 

 them.^ 



That spindles shift during the development of eggs, especially 

 during polar body formation, has been observed by a great num- 

 ber of workers. It has been generally accepted, that this shift- 

 ing was brought about by the protoplasm and not by the asters 

 themselves, but this has been founded on indirect evidence only. 

 The observations of the sea urchin egg allow us, I think, to un- 

 derstand the time and the nature of the movement a little more 

 clearly. The conditions in eggs, in which there is a rotation of 

 the spindle of the second polar body, and eggs which shift from 

 a monaster to an amphiaster are similar in this respect, that we 

 have spindles which must shift to take up new positions. Ordi- 

 narily, this shifting takes place without any bending of the rays 

 of the yoiuig asters in the egg and we have no indication of the 

 cause of the movement. But in monaster eggs, the high devel- 

 opment of the old rays allows us to see where the movement be- 

 gins. An examination of the figures given will show that the areas 

 of movement lie well outside of the centrosphere in the cyto- 

 plasm. Were not old fibers present, there would be nothing to 



^ I have not touched upon the nature of the fibers in the sea urchin egg, in this 

 paper, though the spiral asters throw much light upon this question. In a work 

 shortly to be published, a complete analysis of monaster eggs will be given. I 

 propose to take up in this the nature of the rays. 



THE JOURXAL OF EXPERIMENTAL ZOOLOGY, VOL. 20, NO. 4 



