GENETICS: R. W. HEGNER 
21 
pieces, and these pieces continued to live and reproduce. From these pieces a 
few offspring with one and 2 nuclei were obtained. Specimens with one nucleus 
averaged 82 microns in diameter; and those with two nuclei, 86 microns. 
Here, as in normal specimens, size and nuclear number are closely correlated. 
Other lines of Arcella polypora behaved in similar fashion, but the data 
show that the ratio between nuclear number and cell size differs markedly in 
the different lines. Thus the number of nuclei in line 34 ranged from 5 to 10, 
but the specimens were smaller than were those with a lesser number of 
nuclei in line 5. The specimens in line 5 with 4 nuclei were similar in size 
to those in line 34 with 8 nuclei; those in line 5 with 5 nuclei were about 
as large as those in line 34 with 9 nuclei; and those in line 5 with 6 nuclei, 
approximated in diameter those in line 34 with 10 nuclei. 
This condition suggested the possibility that size in these organisms is con- 
trolled not by the number of nuclei, but by the amount of chromatin within 
the nuclei. Accordingly, measurements were made of the chromatin masses 
in specimen from a number of lines, and this hypothesis was confirmed in a 
remarkable manner. 
Measurements were first obtained of specimens of Arcella dentata. It 
soon became evident that in this species, the larger the specimen the greater 
the amount of chromatin within the nuclei. A chromatin-cytoplasmic mass 
relation was thus established. Measurements were then made of the chroma- 
tin masses in specimens from lines 5 and 34 of Arcella polypora. From these 
measurements, the volumes of the chromatin masses were computed, and they 
were found to be greater in line 5 than in line 34. For example, the total 
volume of the 6 chromatin masses in an average specimen of line 5 proved to 
be approximately equal to the total volume of the 10 chromatin masses of an 
average specimen of the same size in line 34. Similar conditions were found 
to exist when the total volume of chromatin within other specimens of line 5 
was compared with the total volume of chromatin within specimens of the 
same size in line 34. These data prove that the quantity of chromatin con- 
tained in specimens of the same size within the two lines, 5 and 34, was very 
nearly the same, regardless of the variations in the number of nuclei. Thus, 
in Arcella polypora, as in Arcella dentata, there is a certain amount of chroma- 
tin associated with a certain amount of cytoplasm, and a definite chromatin- 
cytoplasmic-mass-relation is shown to exist. 
The data presented above not only show a mass relation between chro- 
matin and cytoplasm, but also a relation between chromatin mass and external 
measurable characters. For example, in Arcella dentata, both the diameter 
of the shell and the number of spines are correlated with the chromatin mass ; 
and in Arcella polypora, the diameter of the shell also depends upon the total 
volume of chromatin. 
A summary of some selection work that I carried on last year with Arcella 
dentata was published in the October number of the Proceedings of this 
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