THE EFFECTS OF CENTRIFUGAL FORCE UPON THE 
ORGANIZATION AND DEVELOPMENT OF THE 
EGGS OF FRESH WATER PULMONATES 
EDWIN G. CONKLIN 
Professor of Biology, Princeton University 
FORTY-SEVEN FIGURES 
In one of his earliest papers Professor Brooks (1879) described 
the early stages in the development of Physa, Lymnaea and Plan- 
orbis, devoting particular attention to the cleavage of the egg, 
the formation of the germ layers and the history of the digestive 
tract. This work was done on living material, observations being 
made upon eggs within the capsules, and he says (p. 77) : 
I have used every effort to actually see the living egg pass through all 
the stages of development. To do this requires such close and constant 
attention that it leaves no time for the more minute study of eggs which 
have been treated with reagents, and I have no observations to offer 
upon the fascinating subject which is now receiving so much attention 
from embryologists — the history of the changes which result from fer- 
tilization, and the origin and behavior of the segmentation nuclei. 
On the other hand the observation of the changes of the living egg from 
one stage to another, brings into prominence certain features which 
would be entirely lost sight of in the study of a series of preserved speci- 
mens. 
Professor Brooks observed that a few minutes after the egg is laid the 
germinative vesicle becomes invisible and that a clear watery fluid ap- 
pears at the pole of the egg at which the polar body is forming. He be- 
lieved that this clear fluid disappeared subsequently during the cleavage. 
Regarding the different substances which may be seen in the egg he 
says (p. 80), ''The end of the second period of rest (4-cell stage) is of 
especial interest, since we find that the segregation of the protoplasm of 
