No. 2.] COMPARATIVE CYTOLOGICAL STUDIES. 
521 
spindle and centrosome of Ascaris, but might it not be com¬ 
pared to the nucleolus alone, or to the nucleolus plus centro- 
somes of the metazoan cell ? However, the significance of 
most protozoan “ nucleoli ” is very problematical. ( Cf . the later 
observations of Lauterborn, ’95a.) 
9. Ontogenetic Origin of the Nucleolus. 
Very few observations have been made to determine the 
mode of origin of the nucleolus, though there are numerous 
hypotheses intended to explain it. We may leave aside, for the 
time being, its mode of reappearance in the daughter-nuclei 
after nuclear division, since a special section will be devoted to 
that subject. 
In order to determine the mode of origin of the nucleolus in 
resting stages of nuclei, I have studied those cells in which at 
first no nucleolus is present, but which after a certain period of 
growth acquire one. Objects well adapted for such investiga¬ 
tion are the ova of the nemerteans and the mesenchym cells 
of Cerebratulus. For details of these processes the reader is 
referred to the observations. 
In the ova of the nemerteans the nucleoli at the time of their 
first appearance are always in close contact with the nuclear 
membrane ; this is also the case for the mesenchym cells of 
Cerebratulus , and probably for the paranucleoli of the ova of 
Rodalia. In all these cells the nucleoli only then leave the 
periphery of the nucleus and wander towards its center, after 
the nucleus has increased more or less in size. There is only 
one explanation for the peripheral position of the nucleoli at the 
time of their first appearance, namely, that their substance is 
extranuclear in origin. This process of formation has already 
been discussed in detail for the several cells, and it is not 
necessary to repeat here all the detailed observations on which 
the main deduction is based. If the nucleolar substance were 
a secretion of the nucleus, as Hacker (’ 95 ) assumes, how would 
this assumption explain the strictly peripheral position of the 
nucleoli when they first arise? For on Hacker’s hypothesis 
we should expect the supposed nucleolar secretions to be de- 
