No. 2.] COMPARATIVE CYTOLOGICAL STUDIES. 
Ehrlich-Biondi stain. The vacuoles are filled with a structure¬ 
less fluid, which stains but lightly. But in four nuclei, the 
sections of which were stained in aqueous solution of methylen 
blue followed by brasilin, a differential stain of the ground sub¬ 
stance was acquired : that pole of the nucleolus which con¬ 
tained vacuoles was stained a bluish green (methylen blue), the 
opposite pole, where no vacuoles could be seen, being of a light 
pinkish color (brasilin), the vacuoles themselves appearing as 
clear unstained spaces (Figs. 17-19). In one nucleus, in which 
two minute nucleoli were present, the one without, the other 
with, a single small vacuole, both nucleoli stained a bluish 
green throughout (Fig. 18). Further, in an unstained nucleus 
fixed with Flemming’s fluid a somewhat similar differentiation 
was visible in the two larger nucleoli (neither of which con¬ 
tained vacuoles), the pole of each nucleolus nearest the nuclear 
membrane being of a deeper color than the opposite pole (Fig. 
11). This differentiation produced by staining would show that 
the ground substance of the smallest nucleoli is homogeneous, 
but that in the larger ones a chemical change takes place in it, 
whereby that portion of the substance opposite the pole where 
the vacuoles first appear differentiates itself chemically from 
that portion of the ground substance lying at the latter pole. 
Unfortunately I had too little material to carry further the 
study of this differentiation. 
In the nucleus is a faintly staining nuclear sap, in which 
irregular granules of various size are massed together espe¬ 
cially near the center of the nucleus ; they do not come into 
contact with the nucleoli, usually leaving a clear space around 
each of the nucleoli (Figs. 7, 8, 11, 14, 17-19). These do not 
stain with haematoxylin or with methylen green, but stain 
red with eosin and brownish red with the Ehrlich-Biondi mix¬ 
ture, in their staining differing little from the substance of the 
nucleoli. With the methylen-blue-brasilin stain mentioned 
above they stain pink, a little more deeply than does the inner 
pole of each of the larger nucleoli (Figs. 17-19). Whether they 
represent physiologically chromatin, or whether they are 
masses of (perhaps nutritive) substance taken into the nucleus 
from the cytoplasm, which might be chemically and genetically 
