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 spindle 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 Zrystphe 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 
