166 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
cases the two processes take place at different times, so that it seems 
probable that chromatin and nucleolus have no fixed relation to each 
other. 
The time of disappearance of the nucleolus is not constant. For 
example, Figures 1 and 2 are from eggs of the same stage, having been 
killed ten hours before the usual time of liberation of the medusa; 
yet in one case the nucleolus is large, apparently having undergone 
little diminution of size, and in the other it has almost disappeared. 
Figures 3 and 4 also represent eggs of the same age; here the differences 
in the size of the nucleoli are still more marked. A comparison of 
these two sets of eggs also makes clear the lack of agreement between 
the disappearance of nucleoli and the appearance of chromatin strands. 
While the eggs are apparently of similar age, the chromatin appears to 
be more concentrated in those cases (Figs. 1, 3) where the nucleolus 
is larger, more diffuse where the nucleolus is smaller (Figs. 2, 4). I 
have no stages intermediate between these four and those in which 
the nucleolus has already disappeared. Figures 5-7 show that when 
the germinative vesicle has advanced further toward the formation of 
polar cells no nucleoli are present. It seems clear from Figures 1-7 
that the nucleolus disappears gradually, perhaps by dissolving rather 
than by breaking up into fragments. 
With regard to the nucleolus, then, the evidence seems clearly to 
lead to these conclusions: (1) it is mainly, if not entirely, a non- 
chromatic body at all stages from the young oécyte to the mature egg; 
(2) it entirely disappears, probably by dissolving, before the nuclear 
membrane has been ruptured; (3) the time of its, disappearance 
varies in different eggs; (4) there is a direct connection between the 
nucleolus and the linin network, so that an exchange of material may 
take place between nucleolus and chromatin along the linin network, 
but not in the form of prepared chromatin. | 
2. Formation of Polar Cells. Before describing the details of 
polar-cell formation something should be said regarding the time of its 
occurrence. In examining living eggs just after they had been dis- 
charged from the medusa (this was done by compressing the eggs 
slightly under a cover glass), the nuclei appeared small and without 
nucleoli. Evidently the polar cells had already been formed, though 
they were no longer discoverable, doubtless because they had been 
lost in the water. Eggs similarly examined a few hours before the 
discharge of the medusae show the germinative vesicle, which usually 
contains a nucleolus. ‘The conclusions reached from the study of 
fresh eggs were confirmed by the examination of sections of fixed eggs. 
