GENETICS: R. W. HEGNER 
19 
QUANTITATIVE RELATIONS BETWEEN CHROMATIN AND 
CYTOPLASM IN THE GENUS ARCELLA, WITH THEIR 
RELATIONS TO EXTERNAL CHARACTERS 
By R. W. Hegner 
School of Hygiene and Public Health, Johns Hopkins University 
Read before the Academy, November 18, 1918. Communicated by H. S. Jennings. 
A problem that has attracted considerable attention during the past twenty 
years is that of the quantitative relations between the nucleus and the cyto- 
plasm of animal cells. 
According to Richard Hertwig, who has been the foremost advocate of the 
nucleo-cytoplasmic-relation theory, a balance between nuclear and cyto- 
plasmic masses exists in the normal cell, this balance being due to the inter- 
change of materials between nucleus and cytoplasm. This state of equilibrium 
may be disturbed by such factors as changes in temperature, over-feeding or 
starvation. The result of these disturbing agents is an excess of nuclear over 
cytoplasmic materials. This excess of nuclear material leads to the depres- 
sion of the cell, which finally ends in death unless normal mass relations are 
re-established in some way. The restoration of the normal equilibrium may 
be attained by the giving up of nuclear material to the cytoplasm, by ordi- 
nary cell division, or by the addition of a foreign element through conjuga- 
tion. Hertwig and others have attempted to account for many of the com- 
plicated stages in the life cycles of Protozoa with this theory, but while this 
hypothesis is apparently applicable to many phenomena it will not bear close 
analysis, and a review of the extensive literature on this subject reveals a fatal 
lack of data on which to base the theory. 
The material that I have used in my investigations consisted of several 
species of shelled fresh-water Protozoa of the genus Arcella. Arcella dentata 
has a shell with tooth-like projections extending out from the periphery. 
This shell averages about 130 microns in diameter and 50 microns in thickness. 
There is a circular mouth opening in the ventral wall of the shell, through 
which protoplasmic extensions are pushed out that serve as locomotor organs 
and for capturing food. The cytoplasmic body within the shell contains two 
nuclei situated on opposite sides of the mouth , opening. These nuclei are of 
the vesicular type, with the chromatin aggregated into a spherical mass in 
the center. Since the shell is almost transparent, especially in young speci- 
mens, it is easy to measure both the entire nucleus and the chromatin-mass 
within it, in the living animal. This makes Arcella peculiarly favorable for 
nucleo-cytoplasmic studies. The usual method of reproduction is binary 
division. When a certain amount of protoplasm has been built up within the 
shell and environmental conditions are favorable, the cytoplasm protrudes 
from the mouth of the shell and secretes a new shell; then, approximately one- 
