PLASMA-STRUCTURE IN EGG OF HYDRACTINIA 223 



consists apparently in the migration of the granules along the 

 linin-threads toward the central point, leaving the lightly staining 

 net merging into the ground-substance. The chromosomes now 

 condense rapidly into small compact tetrads which stain intensely. 

 A number of stages in the formation of a tetrad are shown in 

 figure 24. The nuclear membrane breaks down about this time 

 and the chromosomes which have been scattered through the 

 nucleus collect at the center (fig. 24). 



Up to this point there has been no evidence of a spindle. When 

 it does appear, the chromosomes are already collected in the 

 center of the nuclear area (fig. 35) and the spindle apparently 

 arises in connection with them from the achromatic portion of 

 the nucleus. The spindle, which is many times smaller than the 

 nucleus, differs entirely in structure according to the fixation. 

 The general topography of such a spindle in the center of the 

 nuclear area after Meves' fixation is shown in figm-e 37. The 

 much enlarged spindle shown in figure 38 makes it clear that no 

 centrosomes or astral radiations are present, the blunt spindle 

 lying free in the nuclear area. If material is fixed in sublimate- 

 acetic solution, in addition to the spindle, small asters appear, 

 which are continuous with the coarse net present in the nuclei of 

 such eggs (fig. 36) . They give every appearance of being part of 

 the coagulation phenomena caused by the killing fluid. In some 

 cases a small centrosome-like body occurs at the center of the 

 aster. But since it is not constant and may be asymmetrically 

 placed in regard to the spindle, it also seems to be a result of the 

 coagulation. In fact, I have not been able to find a true division- 

 center, either outside or within the nucleus. The spindle fibers 

 arise, apparently independently of an aster or centrosome, directly 

 out of the nuclear ground-substance, for the spindle appears 

 directly in the center of the large nuclear area, a considerable 

 layer of the nuclear plasma surrounding it. The tetrads are now 

 drawn on to the spindle and become arranged in an equatorial 

 plate (figs. 37, 38). As stated above, the number of tetrads in 

 the equatorial plate has not been definitely determined, the number 

 being between 12 and 15. Since 14 was more commonly present 

 and it is the number appearing in the polar body, it seems probable 



