No. 2.] COMPARATIVE CYTOLOGICAL STUDIES. 449 



covered by the chromatic filament or by lying in a part of the 

 nucleus outside of the plane of the section. In this stage, 

 further, two nucleoli are never present ; accordingly, in the 

 spirem there is neither a disappearance nor a division of the 

 nucleolus. In the dispirem stage each daughter-nucleus contains 

 one nucleolus (Fig. 171), the two nucleoli being, however, often 

 unequal in size. I found very few aster stages, and these were 

 either so unfavorably placed for study or the chromosomes so 

 densely entangled that I could not determine whether a nucleo- 

 lus is present in this stage and whether a division of it takes 

 place at this time. The facts determined are (i) that no divi- 

 sion of the nucleolus occurs in the typical spirem stage, since 

 here only one nucleolus is present ; and (2) that each nucleus 

 of the daughter-spirem has one nucleolus. But I cannot show 

 whether a division of the nucleolus occurs in the time between 

 these two stages or whether the original nucleolus passes over 

 into one of the daughter-nuclei, while in the other one a new 

 nucleolus is produced. In these various mitotic stages the 

 nucleolus usually lies at the periphery of the nucleus, and it is 

 most frequently the case that it is not in contact with the 

 chromatin filament ; it preserves its former shape and staining 

 intensity, and apparently does not decrease in size during the 

 mitosis. To be sure, in the karyokinetic stages under considera- 

 tion it usually appears small in proportion to the size of the 

 particular nucleus, but then it is usually the case in most 

 mitoses, and probably so here, that before the disappearance 

 of the nuclear membrane the volume of the nucleus greatly 

 increases.' 



Two nucleoli, never quite equal in size, are frequently found 

 in certain small nuclei, which the distribution of the chromatin 

 would show to be in a stage at the commencement of the 

 prophasis of the mitosis or at the conclusion of the metaphasis 

 (Figs. 163, 164-167, 170, 172). As the figures show, all these 

 nuclei which contain two nucleoli are more or less of the same 

 size. Nuclei which are a little smaller than these, as well as 

 those which are larger, invariably contain a single nucleolus. 



' The chromatin filament has considerable thickness and is apparently a con- 

 tinuous thread ; it is looped around the inner surface of the nuclear membrane. 



