THE CELL ASTER 495 



consequent to a liquefaction of the cytoplasm in the equatorial 

 plane while elsewhere it remains in the gel state. The lique- 

 faction is accompanied by a loss in the ray-like structure in this 

 region. During the remainder of cell division, a one-sided ap- 

 pearance obtains for each daughter aster, the rays being most 

 prominent at the polar end and absent in the region correspond- 

 ing to the equator of the mother cell (fig. 13). By the time cell- 

 division is completed all of the cytoplasm has returned to the 

 gel state, each cell containing a single aster, the nucleus lying 

 to one side within the sphere. This brings us back to the same 

 figure as in the undivided egg with a single aster (fig. 7). In the 

 segmentation of the blastomeres the process as described is 

 repeated. 



The amphiaster may be twisted in much the same w^ay as the 

 single aster. Figure 12 represents a distortion of the amphi- 

 aster produced by dragging the needle across the equator of the 

 cell and pulling the highly viscous nuclear 'spindle' along with 

 it. On removing the needle, the asters may assume their original 

 appearance. Where a distortion persists abnormalities in cell 

 division occur. 



4. The cell nucleus 



Various suggestions have appeared in print regarding the 

 relation between the formation of the fibers of the nuclear spindle 

 and of the aster. Mention may therefore be made here of some 

 of the results obtained in the microdissection of the nucleus 

 during cell-division. A paper dealing with nuclear structure 

 will be pubhshed shortly. 



The spherical nucleus, which lies between the poles of the 

 amphiaster, possesses a plainly visible nuclear membrane. This 

 has already been described by Wilson ('95) in Arbacia. As the 

 nuclear body becomes bipyramidal (in the metaphase) and 

 elongates (in the ana- and telophases) the boundary between 

 it and the surrounding cytoplasm fades from view. It preserves 

 its identity, however, as a structure which is optically homo- 

 geneous except for the chromosomes which lie in its substance. 

 Manipulation with the needle gives one the impression that it is 



