CELL SIZE AND NUCLEAR SIZE 3 
potency. In such cases differences in the sizes of cell are associ- 
ated with some of the earliest differentiations of the developing 
egg. 
But differences in the size of cells may be due, not to unequal 
cell division, but to unequal rates of division, or to unequal growth 
of cells subsequent to division. In some instances cells divide 
rarely and consequently become large, while adjoining cells divide 
frequently and therefore remain small. The fact that cells are 
not always of the same size at the time of division is one of capital 
importance for it shows conclusively that the factors which bring 
about cell division may be separated from those which cause 
growth. 
In connection with the size of cells as a whole may be considered 
the sizes of many of their constituent parts, such as nuclei, 
chromosomes, plasmasomes, centrosomes, etc. The size relations 
which exist between these parts of the cell and the plasma should 
throw light upon the interrelation between these cell constituents 
in other respects than size. The quantitative relations of differ- 
ent cell constituents at various phases of activity should be of 
significance in the study of many fundamental problems of growth, 
differentiation and cellular physiology. 
Within the past few years several contributions on this sub- 
' ject have appeared, principally from Boveri and R. Hertwig, 
and their students. In so far as these works have dealt with the 
development of the egg they have been based on a study of eggs 
of ‘indeterminate cleavage’ in which it is not possible to trace 
individual blastomeres throughout the cleavage period; the re- 
sults have therefore been mass results, based on averages of 
cells of a given stage. In the study of vital phenomena it is 
frequently important to deal with individual rather than with 
average results; in the following pages I have attempted to apply 
the method of the quantitative study of cells and of cell constit- 
uents to individual blastomeres at various stages of the cleavage. 
