No. 2.] COMPARATIVE CYTOLOGICAL STUDIES. 413 
may conclude, then, that when the nucleus is comparatively 
small, and when no yolk or only small yolk globules are pres¬ 
ent in the cytoplasm, the nucleus derives a nutritive substance 
from the cytoplasm, which is closely similar to that composing 
the youngest yolk globules ; but when the nucleus has grown 
large, and the cytoplasm is packed with large yolk globules, it 
has the power to take up these larger globules also. 1 
To return, then, to the second stage of nucleolar differentia¬ 
tion. This stage does not commence when the nucleolus has 
attained a certain size, but may commence in some nucleoli 
earlier than in others; and again it is not marked by a particular 
stage of development of the yolk in the cytoplasm. The fluid 
vacuoles probably stand in a genetic relation to the small 
nutritive globules found in the nucleus, which have been 
just described. That is, these globules of the nucleus pene¬ 
trate into the nucleolus and then constitute the fluid vacuoles 
of the latter structure. I have reached this conclusion after 
observing that the vacuoles of the nucleolus and the small 
nutritive globules within the nucleus always stain in exactly 
the same way. This assumption is further strengthened by 
the fact that, when the nutritive globules lie in the nuclear 
sap at some distance from the nucleolus, they have invariably 
a spherical form ; but in those numerous cases where they may 
be seen apposed to the outer surface of the nucleolus they 
become flattened against the surface of the latter, as if the 
nucleolus were (figuratively speaking) a loadstone which 
attracts them to itself (Figs. 63, 69, 75). If this origin of 
the vacuoles of the nucleolus were not the true one it would 
be difficult to explain their mode of genesis, since there appears 
to be no other substance within the nucleolus from which they 
could be derived, and there is no reason for supposing that the 
1 The intensity in the staining of the yolk globules increases with their size, and 
the largest stain much more deeply than does the nucleolus. During all the 
earlier growth stages the nuclear membrane is retained, and it is seldom, and 
then only slightly, irregular in outline; therefore the yolk cannot be taken up by 
the mechanical aid of amoeboid processes of the nucleus, but its substance must 
osmotically penetrate the nuclear membrane. And as I mentioned above, it does 
not seem probable that the yolk globules retain their shape while penetrating this 
membrane, but diffuse through it in the form of an irregular fluid mass, and then 
in the nucleus this fluid becomes re-formed into globules. 
