4 H 
MONTGOMERY. 
[Vol. XV. 
substance of these vacuoles is a differentiation of the nucleolar 
ground substance. We may assume, then, that this explana¬ 
tion of the genesis of the nucleolar vacuoles is the correct one, 
and now proceed to explain the changes in the nucleolus dur¬ 
ing the successive development of its vacuoles. If we take the 
size of the nucleolus as a general criterion (though it is not an 
infallible one, since there are considerable individual differences 
in different nucleoli (cf. Figs. 62, 65, 80)) of the stage of 
the nucleolus, the process of assimilation of the nutritive glob¬ 
ules from the nucleus by the nucleolus seems to be in general 
as follows : first, one or two globules are taken into the nucleo¬ 
lus, and later when others (apparently a varying number) are 
also taken up into it, we reach a stage when the nucleolus 
contains a number of fluid vacuoles (the assimilated nutritive 
globules) (Figs. 64 and 70). Then these vacuoles commence to 
fuse together (Figs. 63, 66, 72), finally by their fusion giving 
rise to one large vacuole, which fills about three-quarters of the 
space of the nucleolus, and always lies excentrically within the 
nucleolus (Figs. 68, 69, 73, 77, 79). The nucleolus has now 
attained its greatest dimension and is either perfectly spher¬ 
ical, or more usually ovoid in shape. Its large excentric vacuole 
is encircled by a peripheral layer of the primitive homogeneous 
ground substance of the nucleolus, which has undergone no 
structural or chemical change. This layer of ground sub¬ 
stance becomes necessarily thinner as the vacuole becomes 
larger, i.e., as the pressure from within becomes greater. But 
since the large vacuole lies peripherally, the peripheral sub¬ 
stance of the nucleolus remains thickened at that point opposite 
the vacuole, and this thickened portion of the nucleolar wall 
has most frequently the form of a concavo-convex lens (or on 
a cross-section, of a half moon), the concave side of which 
borders upon the vacuole. This thickened part, as the remain¬ 
ing portion of the peripheral layer of the nucleolus at this stage, 
is in every respect identical with the ground substance of the 
nucleolus in earlier stages, before vacuoles had made their 
appearance in it ; and the total amount of the substance of the 
peripheral layer seems to be equal to the amount of the homo¬ 
geneous substance of the nucleolus at the end of the preceding 
