CELL SIZE AND NUCLEAR SIZE 3y5) 
the early cleavages. These results show a general agreement 
with those of Godlewski. 
The growth of plasma at the expense of yolk during the matur- 
ation and the cleavage period, was shown to occur in my studies 
of the effects of centrifugal force on the eggs of Lymnaea and 
Physa (Conklin, 710). In the living eggs of these animals the 
substances may be stratified by centrifugal force into a gray 
(light) zone, a clear (middle) zone and a yellow (heavy) zone; the 
eray and clear zones constitute what I have here regarded as proto- 
plasm, while the yellow zone is in large part composed of yolk. 
“Before the first maturation the yellow substance composes at 
least one-half of the entire egg; just before the first cleavage it 
composes only about one-eighth of the egg. The clear and gray 
substances, which together constitute about one-half of the egg 
in the earlier period, form seven-eighths of the egg in the later 
period,” (p. 436). 
In the normal eggs of Lymnaea and Physa, which have not been 
centrifuged, the clear and yellow substances are easily recog- 
nizable, and the stages in the transformation of the latter into 
the former have been studied in the paper mentioned, from which 
the following summary is quoted: 
In the course of development, from the maturation of the egg to the 
gastrulation, the relative quantities of clear (plasma) and yellow sub- 
stance (yolk) 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 sub- 
stance) 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 develop- 
ment, but in much greater part to the transformation of yellow sub- 
stance (yolk) into the clear (plasma). 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 gradually 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 (p. 423). 
