256 Merkel Henry Jacobs 



eggs produced. In one such case thirty rotifers which had laid no 

 eggs for some time previously in less than a week produced fifty- 

 five eggs. Similar results were obtained in other experiments. 

 The length of time necessary for the appearance of eggs was about 

 the same as that required in the large cultures. 



A factor which must be considered as a possible cause of the 

 phenomenon just described is the food supply. Associated with 

 Philodina there are always found large numbers of the unicellular 

 plant, Sphaerella lacustris. The rotifers feed to some extent on 

 the partly grown cells but much more largely on the small micro- 

 zooids which are produced in large numbers after each period 

 of desiccation. It might be thought, therefore, that the effect 

 observed was due not to the direct influence of the desiccation 

 but to its indirect influence in causing an increased food sup- 

 ply. To decide this point, experiments were tried in which the 

 rotifers after being dried were placed in water free from micro- 

 zooids. In all such cases, although there was no increase in the 

 food supply, egg production was brought about just as before. 

 The following experiment is a typical one selected from a number 

 showing similar results. 



Six rotifers were taken from a small culture in which no eggs 

 had been laid for several weeks, and dried on a clean piece of 

 filter paper for twelve days. On March 7 they were placed in a 

 dish with water and as soon as they had revived the filter paper 

 was removed. No food of any sort was present. In one of the 

 rotifers the ovary was slightly enlarged, in the others it showed no 

 signs of developing eggs. On March 8 the alimentary tract of all 

 had acquired a glandular appearance but contained no food. 

 Traces of egg formation were observed. On March 9 three con- 

 tained fairly well developed eggs. On March 10 all six contained 

 eggs and two individuals contained two eggs — a rather unusual 

 occurrence. On the i ith one of the rotifers had died but the other 

 five all contained eggs. On the 12th seven eggs had been laid, 

 on the 14th eleven and on the i6th twenty-one, of which four had 

 hatched. By the 17th ten of the twenty-one eggs laid l^ad hatched 

 and by the 21st all had hatched, no new ones being laid after March 

 16. On the 2ist only one of the five rotifers showed any signs of 



