NUCLEAR DIVISION. 9 



and frequently forms a dense mass. These phenomena indicate 

 clearly that chromatin and centrosphere are in direct communication 

 through the nuclear membrane. The first step in the division is 

 characterized by the appearance of a well-developed aster or system 

 of radiations about the centrosome. It seems very probable here that 

 the radiations grow out into the cytoplasm from the centrosome as a 

 center. In the development of the radiations the nucleus probably 

 cooperates. At this stage the chromatin is contracted into a dense 

 net toward the centrosphere and appears in close connection with it. 

 From the chromatin mass several fine achromatic threads extend 

 toward the nuclear membrane (Fig. 4, B). 



In the next stage observed, the two poles of the spindle have been 

 formed, which lie some distance apart on the nuclear membrane 

 (Fig. 4, C). The polar radiations are well developed, and from each 

 centrosome a cone of spindie fibers extends into the nuclear cavity. 

 The diverging fibers seem to be inserted in the nuclear membrane at 

 points opposite the centrosome. As in Dictyota the two systems of 

 fibers cross each other at nearly right angles without in any way 

 uniting. Whether the two centrospheres arose by a division of the 

 primary centrosphere cannot be stated with absolute certainty, since 

 the intermediate stages between B and C, Fig. 4, were not observed, 

 yet from what is known in Stypocaulon and in Dictyota, it seems 

 reasonable to suppose that the centrosphere may undergo a division 

 in Erysiphe also. 



The chromatin, at this stage, seems to be reduced in mass to that 

 which will appear in the nuclear plate. It lies distributed in irregular 

 lumps among the fibers opposite the two poles. The nucleolus has 

 now disappeared, or, in some cases, it may remain in the form of a 

 weakly staining residue. The spindle fibers within the nucleus be- 

 come attached to the chromosomes and then contract strongly, bringing 

 the chromosomes into the center of the nuclear cavity (Fig. 4, C, D, 

 E, F). Some of the fibers of the bent spindle appear, at this stage, to 

 extend uninterruptedly from pole to pole. The continuous fibers are, 

 in all probability, formed by the union of those which are not attached 

 to the chromosomes. 



The polar radiations now undergo a marked change, becoming shorter 

 and thicker, as if drawn in toward the poles. The majority of the 

 radiations diverge only slightly. They are contracted into bundles or 

 brush-like collections, which stand perpendicular to the surface of the 

 nucleus. Some of these radiations, however, diverge somewhat from 

 the central group, but all the polar radiations are not centered upon a 

 single point. The pole of the spindle is exactly as broad as the base of 



