CELL SIZE AND NUCLEAR SIZE tS 
elaboration and further extension of that work. The egg of 
Crepidula plana is a particularly favorable object for the study 
of such a subject. The eggs may be stained and mounted entire 
in such manner that all of these cell constituents show with great 
distinctness, and the advantage of seeing whole eggs and nuclei 
in making such measurements is sufficiently obvious. 
A further advantage of the study of whole eggs is found in the 
fact that the exact stage in the cell cycle is more easily determined 
in whole eggs than in sections. My work has shown that it is 
most important in comparing the sizes of cell constituents to 
compare precisely corresponding stages, and accordingly I have 
chosen for measurement stages of the maximum and minimum 
sizes of the nuclei. The growth of the nucleus is more rapid in 
the last stage of the resting period preceding mitosis (‘ Kernteil- 
ungswachstum’ of Hertwig) than at any other time in the cell 
cycle, and in order to find the maximum nuclear size it is neces- 
sary to measure the nuclei just before the nuclear membrane dis- 
appears. Such stages are easily selected by looking for eggs in 
which part of the nuclei of a certain generation of cells are divid- 
ing while others have not yet begun to divide, as in figs. 1 and 2. 
At this stage there is great uniformity in the dimensions of the 
nuclei of particular blastomeres, and as the nuclei at this stage 
are regular spheres, it is easy to calculate their volumes. 
The cell dimensions are more difficult to determine than are 
those of the nucleus. In cells which contain yolk and in cells of 
irregular shape it is not possible to determine the volume of the 
plasma with accuracy. After the first cleavage the plasma and 
yolk are sufficiently well separated so that the dimensions of the 
cytoplasm can be fairly well observed; before the first cleavage the 
plasma is so mixed with the yolk that this can not be done and I 
have here had recourse to the method of centrifuging the yolk 
out of the egg, leaving only the nucleus and plasma which can 
then be easily measured. Wherever it could be done, I have 
chosen cells for measurement which were as nearly spherical as 
possible, but where the dimensions in different axes differed con- 
siderably I have determined the mean diameter, which is the one 
recorded. 
