CHROMOSOMAL VKSICLES IN PIIASKOLUS. 215 



structure, where the four triangles come together, consists of non- 

 vacuolated chromatin and makes up a part of the nucleolus. 

 The outer part is principally achromatic and, consequently, 

 much larger. Thus, every chromosome contributes to the forma- 

 tion of the nucleolus and 'likewise to the so-called nuclear reticu- 

 lum. As stated above in distinguishing between interphase and 

 rest, when the meristematic region moves forward leaving behind 

 it cells that cease dividing we find an increase of scattered chro- 

 matin and a decrease in the size of the nucleolus. Since every 

 chromosome enters into the formation of the nucleolus, this giving 

 up of material by the latter body affects all of them and inasmuch 

 as the nucleolus remains practically spherical the chromosomes 

 apparently are equally active. 



Prophase. Before attempting to interpret the structure of the 

 resting nucleus it is necessary that one study the development 

 of that body in the telophase. It is equally necessary that one 

 check these results by studying the behavior of the chromosomes 

 during the prophase. 



The onset of mitosis is first indicated by an increase of size and 

 of the basophilic character of the scattered chromatin granules 

 (Fig. 14). The next evident change is the appearance of con- 

 siderable quantities of chromatin on the linin strands (Figs. 15). 

 A little later the nucleus becomes smaller and consists almost 

 entirely of chromatin. The linin strands are now covered with 

 heavy bands of chromatin and the intervening spaces are well 

 filled with large chromatin granules (Fig. 16). 



The next change is important and introduces a very revolu- 

 tionary fact. The nuclear membrane does not dissolve and dis- 

 appear as has been universally described previously. Breaks 

 appear in its wall, but each break leads into a short channel which 

 terminates before reaching the general outline of the nucleolus, 

 and this channel is bordered on either side by the end of a chromo- 

 some, now fully formed except that one end is still imbedded in 

 the nucleolus. Fig. 17 shows a cell in which one side of the 

 nuclear membrane is still intact and one can see that it appears 

 to be continuous with the chromosomes projecting from the 

 nucleolus on the other side. 



The spherical outline of the nucleus entirely disappears in the 



