No. 2.] COMPARATIVE CYTOLOGICAL STUDIES. 447 
nective-tissue cells, lead to the conclusion that the nucleolus 
first appears in the young germinal vesicle, and more particu¬ 
larly, that the substance or che nucleolus is extranuclear in 
origin, and stands in a genetic relation to the substance of the 
young yolk balls. The substance of both is homogeneous and 
stains identically; by fixation in Hermann’s fluid, followed by the 
triple stain of Flemming, the nucleolus and the yolk balls stain 
a brownish yellow (Fig. 160); by fixation in corrosive sublimate 
and staining in haematoxylin and eosin both structures are 
colored a yellowish red (Fig. 177). Still more conclusive is 
the following observation : while the greater number of the 
yolk balls may lie at some distance from the nucleus, one or 
several are very frequently in close contact with the outer sur¬ 
face of the latter, and yolk balls may even be found which are 
halfway through the nuclear membrane, or which have com¬ 
pletely transversed it and lie within the nucleus (Fig. 160). 
Thus the nucleolus would seem to owe its origin to the sub¬ 
stance of yolk balls which have been taken into the nucleus. 
The very marked increase in the size of the nucleus and the 
nucleolus is probably caused by a continued process of yolk- 
ball assimilation on the part of the nucleus. This may be 
observed in numerous cases where small globules of yolk- 
ball substance lie within the nucleus, some at its periphery or 
close to the nuclear membrane, others flattened against the 
nucleolus (Figs. 160 and 177). By the use of the haematoxylin- 
eosin stain the nucleolar substance usually stains a little more 
intensely than the substance of the yolk balls (Fig. 177); this 
would show that this substance, after being taken up by the 
nucleus, undergoes a chemical change within the latter. Those 
yolk balls which are not assimilated by the nucleus remain in 
the cytoplasm and give rise to the yolk globules, as has been 
described. Thus the nucleolus probably has an extranuclear 
origin and represents a portion of the yolk-ball substance taken 
into the nucleus; its rapid increase in size is due to the addition 
to it of other similarly assimilated globules of substance. 
In the largest germinal vesicles seen (though these were 
not mature) the nucleolus is usually spherical in form, seldom 
oval, and homogeneous in structure, except that it sometimes 
