INTRODUCTION. 



tance. Near the nucleus the cytoplasm is more granular, with smaller 

 meshes. It is more nearly a thread-like net- work than alveolar in 

 structure, and appears with differential staining as kinoplasm. This 

 very fine granular thread-work often extends in among the radiations 

 of the centrosphere. 



The resting nucleus shows a large vacuolated nucleolus and a fine 

 linin-reticulum with rather large meshes, upon which are arranged 

 small and nearly uniform granules, all of which do not react as 

 chromatin. With the advance of karyokinesis, the chromatin begins 

 to collect into larger and somewhat irregular masses that finally become 

 the chromosomes. There is not developed, as in vegetative cells of 

 this plant, a regular and uniform chromatin spirem or ribbon. The 

 nucleolus becomes more vacuolated and soon disappears. The nuclear 

 cavity presents a more granular appearance, the granules staining 

 more densely. 



The kinoplasmic fibers now penetrate the membrane of the nucleus 

 and enter its cavity, while at the same time the polar radiations seem 

 to diminish in number (Fig. i, C). On entering the cavity some of 

 the fibers proceed in advance of the others. Some pass straight to- 

 ward the center of the nucleus, while others diverge toward the sides. 

 As these fibers approach from opposite sides of the nucleus, they tend 

 to collect the chromosomes into an irregular mass in the equatorial 

 region, where they finally form the nuclear plate (Fig. i, D). Cer- 

 tain of these fibers coming from opposite sides seem to unite at their 

 ends to form the continuous spindle fibers which extend from pole to 

 pole ; others fasten themselves to the chromosomes, and still others 

 diverge toward the nuclear membrane in the equatorial region (Fig. 2, 

 E). In the mature spindle, therefore, the fibers present the following 

 orientation : those radiating from the poles, the continuous spindle 

 fibers extending uninterruptedly from pole to pole, those running from 

 the poles to the chromosomes, and the fibers which diverge from the 

 poles toward the equatorial region and end in the cytoplasm (Fig. 2, F) . 

 The nuclear membrane in the tetraspore mother-cell of Dictyota 

 disappears very gradually during the process of karyokinesis, often 

 persisting at the sides when the spindle is mature (Fig. 2, F). It begins 

 to disappear at the poles as soon as the fibers enter the nuclear cavity, 

 and by the time the anaphase is reached no part of the membrane can 

 be distinctly seen. Thus the spindle, with the exception of the polar 

 radiations, lies within the nuclear cavity, its fibers, however, being 

 largely of cytoplasmic origin. To what extent any nuclear substance 

 contributes to the formation of the spindle is difficult to determine. 

 On the disappearance of the nucleolus, numerous granules appear in 



