FOOD AND PARTHENOGENETIC REPRODUCTION. 1 95 



The method used for deciding whether the races had maintained 

 their original vitality or whether they had decreased in their 

 constitutional vigor has been the rate of parthenogenetic repro- 

 duction. In order to determine the comparative vigor of the 

 races A and B, their rates of parthenogenetic reproduction were 

 obtained by counting the young produced by a mother during a 

 certain period of time for . several successive generations at 

 intervals of about thirty days. 



The first comparison was made by counting the young daugh- 

 ter-females of both the races A and B from generations 22-29. 

 The second comparison was made by counting the young in the 

 same manner as above from the generations 43-47. The third 

 was made by counting the young from generations 61-67. No 

 green food was given to race B during the generations when 

 these counts were made so that conditions would be identical 

 at the time the rate of parthenogenetic reproduction was being 

 determined. 



During the above named generations fifteen daughter-females 

 of approximately the same size of each race were isolated. This 

 was done in order to give a greater number from which to get 

 the average of young females produced, thereby decreasing the 

 chance of error. 



When the third and also the last comparisons were made 

 fifteen females from various wild races of Hydatina senta were 

 isolated and placed in separate watch glasses. These wild 

 females have been called race C. This race was fed only pure 

 culture of Polytoma. It was kept for only a few generations, 

 being conducted under identical conditions and parallel to 

 generations 65-67 and 169-175 of races A and B. This intro- 

 duction was made in order to make a comparison of the rate of 

 parthenogenetic reproduction between races A and B and various 

 wild races taken from their normal environment. 



Results. 



The first series of observations on the rates of parthenogenetic 

 reproduction in these two races was started at the beginning of 

 the twenty-second generation. The results at this time as can 

 be seen in Table I., are extremely variable. Only five daughter- 



