No. 2 .] COMPARATIVE CYTOLOGICAL STUDIES. 
the nodal points of a “ Wabenwerk ” in the sense of Biitschli). 
This intracellular membrane is not open at any point, and 
a longitudinal section of it shows it to be not spherical but 
oval in outline, the apices of the oval being furthest removed 
from the nucleus. It is present only in the second stage of the 
nucleolus, and between it and the nucleus no yolk balls occur. 
I have never seen such a structure in any other egg cells except 
in the ova of Gryllus abbreviatus ; a similar structure was found 
by van Bambeke (’ 83 , eggs of Leuciscus , Lota), Shafer (’ 80 , egg 
of Lepus ), and Gerould (’ 96 , Caudina egg). 
4 . Tetrastemma catenulatum (Verr.) Montg. 
(Plate 23 , Figs. 103 - 133 ; Plate 24 , Figs. 137-139) 
The formation of the yolk may be spoken of first, then the 
nucleoli proper, and afterwards certain large nuclear structures 
which may or may not represent nucleoli of another kind. 
The yolk first appears in the form of one or two yolk balls 
( Yk. Bl., Figs. 107, 108, 112, 114-116) in the cytoplasm; 
the larger ones are regularly oval as a rule, and the smaller 
ones spherical. A number of these yolk balls are produced 
successively in each cell, and by their fragmentation the ulti¬ 
mate yoke spherules ( Yk. Gl.) are evolved. Each such ball is at 
first smaller than the nucleus of the cell in which it occurs, but 
gradually increases in size, though the maximum size which it 
may attain is not a fixed quantity, but is quite variable. As it 
increases in size it also gradually becomes more deeply stained, 
attaining its most intense staining when it has attained the 
limit of size. The substance of these balls is dense, finely 
granular, not brittle, somewhat refractive ; in the youngest 
stages of their formation they often appear nearly homogeneous. 
About the time a ball has reached its maximum size it com¬ 
mences to change both structurally and chemically, vacuoles 
appear in it, it begins to stain less intensely, and becomes 
irregular in outline. Thus it becomes either coarsely granular, 
or else unstaining vacuoles appear scattered through it, and 
with eosin stains no longer a deep red, but a light red or even 
yellowish. Next it breaks into a number of pieces, whereby 
