428 ROBERT W. HEGNER 



number were shown to be closely correlated. It was also discov- 

 ered that a definite relation exists in these animals between 

 nuclear number and size and between chromatin mass and cyto- 

 plasmic mass. These facts led to the experiments on under- 

 feeding described below. The data obtained, furnish infor- 

 mation regarding fission rate and variations in diameter of the 

 shell. 



a. The effects of underfeeding upon the rate of fission 



Specimens of Arcella dentata were taken from cultures that 

 were being used for selection experiments, and placed in a medium 

 consisting of one-half distilled water and one-half filtered pond 

 water. Instead of a gradual decrease in nuclear and cytoplasmic 

 material and the cessation of reproduction as was expected, sev- 

 eral specimens proceeded to divide, and within a month three 

 rather large families had been produced by them. Evidently, 

 even after being filtered, enough food remained in the pond 

 water to enable the arcellas to grow and reproduce. However, 

 the amount of food present was much less than was ordinarily 

 given to the specimens in laboratory cultures, and several in- 

 teresting results appeared which were apparently due to under- 

 feeding. 



There was a marked retardation of the division rate during 

 the period when the specimens were underfed. Fission occurred 

 at intervals of from two to ten days, with the following means: 



Family 2. Average interval between fissions, 5.25 days. 

 Family 3. Average interval between fissions, 3.47 days. 

 Family 4. Average interval between fissions, 4.15 days. 



In contrast to this, the division rate of the parental lines 

 under normal cultural conditions was approximately 2.50 days. 

 Previous work on pure lines in Arcella dentata indicated that 

 different lines differ in division rate, but these were all being 

 reared under similar conditions and supplied with an abundance 

 of food. The differences in division rate between these pure 

 lines were, therefore, not due to differences in nutrition. The 

 results just described, however, show that it is necessary to keep 



