EFFECTS OX EGGS OF FRESH WATER PULAIONATES 423 
to all of these cells (fig. 22). Finally as the ectoderm cells of clear 
protoplasm overgrow the endoderm and mesoderm cells, the 
yellow material is entirely removed from the surface. 
In the course of development, from the maturation of the egg 
to the gastralation, the relative quantities of clear and yellow 
substance are reversed. At the beginning the clear substance is 
small in quantity, and is chiefly visible in the germinal vesicle, 
(though experiments show that some of it is distributed through 
the yellow substance) and at this stage the entire cell body is 
yellow in color. With the establishment of the germinal layers 
the yellow substance is limited to the few cells constituting the 
endoderm and mesoderm, while all the rest of the embryo, by far 
the larger part, is composed of clear substance. This change in 
the relative quantities of these two substances is due in part to 
their separation and segregation during the course of development 
but in much greater part to the transformation of 3'ellow sub- 
stance into clear. 
It is a phenomenon of general occurrence among many animals 
that the clear protoplasm of the egg is very small in quantity 
before the dissolution of the germinal vesicle and that it gradu- 
ally increases in quantity after that stage. This is doubtless 
due in large part to the dissolving of yoLk and its conversion into 
clear protoplasm, and it is a significant fact that this process 
takes place most rapidly after the breaking down of the wall of the 
germinal vesicle and the escape of a large part of the nuclear 
contents into the cell body. Whether the clear substance which 
normally goes into the ectoderm cells, and the yellow material 
which constitutes the endoderm and mesoderm cells are totipo- 
tent, or specifically differentiated histogenetic materials can be 
determined only b}^ experiment. 
EXPERIMENTS 
Preliminary observations on the effects of pressure on these 
eggs were made in 1902, but no extended experiment were under- 
taken until 1909. From April 3 to June 2 of that year a large 
number of experiments were made on the eggs, of Physa ancillaria 
