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MONTGOMERY. 
[Vol. XV. 
Third nucleolar stage. —The nucleoli increase in number, but 
gradually become smaller and wander towards the periphery 
of the nucleus (Figs. 154 and 155), until they all lie close to 
the inner surface of the nuclear membrane. In this stage 
they attain their maximum staining intensity, as is well seen 
after the use of Heidenhain’s iron haematoxylin, by which 
they become colored a greenish blue (Fig. 157), while in the 
previous stages they are brownish yellow, unstained by the 
haematoxylin. 
Fourth nucleolar stage. — Vacuoles of varying size arise in the 
nucleoli, and become somewhat irregular (instead of spherical) 
in outline (Figs. 156 and 158). In numerous nuclei it may 
be noticed that all the nucleoli lie close to the nuclear mem¬ 
brane, except a single one, which is placed nearer the center 
and differs from the others in not staining with haematoxylin, 
though it usually contains vacuoles ; it may be a nucleolus 
which has not developed as fast as the others have (Fig. 
156). 
All nucleoli in the third and fourth stages are very uniform 
in size, and smaller and much more numerous than in the 
second ; since there are no facts which permit us to conclude 
that new nucleoli are being formed in the last two stages we 
must consider that in them a division of the nucleoli must take 
place, and this would explain their increase in number and 
concomitant decrease in size. The fourth stage would seem to 
be characterized by the commencement of a degeneration of 
the nucleoli, if the presence of vacuoles and the irregularity 
of form may be taken as a criterion of degeneration. Neither 
in this species nor in the other nemerteans examined have I 
seen stages showing the formation of the pole spindle, so that 
I cannot describe the ultimate fate of the nucleoli. But the 
observations of those who have studied these divisions seem 
to show that they all disappear before the pole spindles are pro¬ 
duced ; and accordingly the phenomena characteristic of our 
fourth nucleolar period might represent the commencement of 
these degenerative processes. 
The method of formation of the yolk may next be considered, 
since the yolk stands in a certain relation to the genesis of the 
