No. 2 .] COMPARATIVE CYTOLOGICAL STUDIES. 527 
Sync/iytiiuni) ; Reinke studied the mitosis of the spleen cells 
of the mouse, and found that the single parent-nucleolus divides 
into three or four pieces, while at the end of the mitosis each 
daughter-nucleus contains a single nucleolus. In the mitoses 
of the ovogonia of Linens and Polydora my own observations 
show that the nucleolus persists in the nucleus, and each 
daughter-nucleus contains one nucleolus, so that it is very 
probable that in these cases the parent-nucleolus divides into 
two, and each daughter-nucleus thereby receives a half of it ; 
but these mitoses were so small that I was unable to decide 
this point definitely. Rosen (’95) finds nucleolar division in 
root cells of Phaseolus ; J. Wagner (’96a) describes a similar 
division of a “ nucleolus ” in spermatocytes of Arachnids, though 
this case, like that described by Henking (’90), probably repre¬ 
sents a chromatin nucleolus. This persistence of the nucleolus 
in the nucleus during mitosis must be considered atypical. 
(b) The nucleolus disappears during mitosis. — This is the 
most usual mode of behavior of the nucleolus during mitosis. 
The nucleolus either gradually diminishes in size, and so finally 
vanishes, or else it first fragments into a number of smaller 
pieces, and then these disappear. The only cell which I had 
for the study of this phenomenon was the ovum of Piscicola 
during the formation of the first pole spindle. When this 
spindle is complete no trace of nucleolar substance is to be 
seen anywhere in the cell. In stages immediately antecedent 
to that of the spindle, numerous minute granules, as well as a 
smaller number of larger globules, are dispersed through the 
nuclear sap; all these stain with eosin, and I regard them as 
particles of nucleolar substance which had become separated 
from the nucleolus. Thus a dissolution of the nucleolar sub¬ 
stance commences before the nuclear membrane has disap¬ 
peared, and after this membrane has vanished it is probable 
that all the nucleolar substance must be dissolved by the action 
of the cytoplasm, or at least become dispersed through the 
latter, so that no remnant of it is to be found in the region of 
the spindle or of the chromosomes. During the process of 
dissolution of the nucleolar substance in the nuclear sap the 
chromatin elements stain red (with eosin), and this fact may be 
