20 
GENETICS: R. W. HEGNER 
half of the protoplasm passes back into the old shell, and the two shells sepa- 
rate. It is thus possible to distinguish between parent and offspring. 
The experiments performed on pure lines of Arcella dentata prove that a 
definite relation exists between nuclear number and cell size within each pure 
line. 
One of the members of line 150 was cut in two so that each half contained a 
single nucleus. Both halves continued to live and reproduce; the offspring 
were slightly irregular in shape, but the spines could be counted easily and 
the diameter measured. These offspring were smaller than the original parent 
and possessed fewer spines. They were each provided with only one nucleus. 
From these two uninucleate half-specimens were derived 209 uninucleate de- 
scendants which had a mean spine number of 11 and a mean diameter of 
116 microns. 
After a number of generations that differed in different cases, all of the 
uninucleate specimens produced empty shells and became binucleate again. 
Apparently during this process the single nucleus divided into two, as in ordi- 
nary division, but the new shell that was formed, was cast off empty, and all 
of the cytoplasm and both nuclei were retained in the parent shell. The 
binucleate specimens thus formed, gave rise at once to binucleate offspring. 
These offspring were larger than the parent; and their offspring were still 
larger. In this way a gradual increase in diameter and in spine number 
took place generation after generation, until at the end of the third or fourth 
generation, an equilibrium was established and a mean diameter and spine 
number were regained characteristic of the line before the experiments were 
begun. Thus in line 150, the size of the organisms and the characteristics 
of the shell depend upon the number of nuclei, and each nucleus is accom- 
panied by a rather definite quantity of cytoplasm. 
Other lines of Arcella dentata were subjected to this and other kinds of 
operations and the data obtained fully confirm the conclusion just stated. 
Arcella pol3^ora differs from Arcella dentata in the absence of spines and 
in the greater number of nuclei. In this species the nuclei are distributed 
at approximately equal distances from one another. It was found that 
within a line derived by fission from a single specimen, the number of nuclei 
varied. In line 5 they varied from 3 to 7. When the diameters of these 
specimens were compared with the number of nuclei they contained, a remark- 
ably high correlation was revealed. The mean diameter of specimens with 
3 nuclei was 109 microns; of those with 4 nuclei, 113; of those with five nuclei, 
120; of those with six nuclei, 127; and of those with seven nuclei, 130 microns. 
It is evident that as the number of nuclei increases the size of the organism 
increases, and that in this species as in Arcella dentata, a rather definite 
amount of cytoplasm accompanies each nucleus. No specimens with less 
than 3 nuclei appeared in the cultures, so operations were resorted to, in order 
to obtain individuals with one and two nuclei. Specimens were cut into 
