No. 2.] COMPARATIVE CYTOLOGICAL STUDIES. 531 



of the nucleolar substance is probably different in different 

 objects. 



We have found above that in the simplest though secondary 

 nuclear divisions, the amitotic, the nucleolar substance of the 

 parent-cell is transported into the daughter-nuclei by the me- 

 chanically simplest process, namely, by a direct division of the 

 parent-nucleolus ; this is very frequently the case in amitosis, 

 though it does not always occur. But in most mitotic divisions 

 the nucleolus first disappears, i.e., there would seem to be an 

 indirect mode of transference of its substance corresponding to 

 the indirect mode of transference of the chromatin and linin ele- 

 ments. Now all mitotic divisions do not proceed on exactly the 

 same plan, for we find differences in regard to the presence of a 

 central spindle, in regard to the number of the chromosomes, etc. 

 Accordingly, one would expect also different modes of transfer- 

 ence of the nucleolar substances. Thus in some cases, as 

 Wager ('93) suggests, the chromosomes may serve as mechan- 

 ical vehicles for the transportation of this substance. In many 

 other cases it is very probable that this substance, after the 

 disappearance of the nuclear membrane, becomes dispersed in 

 the cytoplasm ; and then each of the daughter-nuclei may 

 either take up this substance from the cytoplasm again, or may 

 produce its own nucleolus from a new substance, owing to the 

 primitive nucleolar substance having been assimilated by, or 

 even discharged from, the cytoplasm. There are observations 

 in support of each of these three modes of re-formation of 

 nucleoli in the daughter-nuclei. But since when the nuclear 

 membrane disappears the cytoplasm probably comes into con- 

 tact with the substance of the nucleoli, it is most probable that 

 it would produce either a physical or a chemical change in the 

 latter, and hence the second and third modes would appear 

 the more probable. Accordingly, I agree with Humphrey ('94) 

 that there is no substantial basis for Zimmermann's ('93) con- 

 clusion "omnis nucleolus e nucleolo," or more strictly speaking, 

 that the nucleolus in most cases is not derived from a previously 

 existing one. But the third mode of diffusion of the nucleolar 

 substance is in reality not a transference of this substance at 

 all, since it probably becomes lost in the cytoplasm ; and hence. 



