ENVIRONMENT ON ARCELLA 431 



The mean diameter of the Une from which the progenitor of 

 family 2 was taken was 26.40 units and the mean spine number 

 10.90. The mean diameter of the line from which the progenitors 

 of familes 3 and 4 were taken was 34.00 units and the mean spine 

 number 14.59. 



In connection with these experiments in underfeeding it may 

 be noted that a large majority of wild specimens, when collected 

 from the vegetation in ponds, possess very little cytoplasm. This 

 is probably due to the struggle these minute organisms must 

 undergo in their natural habitat. On the other hand, it is not 

 unusual after being brought into the laboratory for the offspring 

 of such wild specimens to average much smaller than their orig- 

 inal progenitors, although they are supplied with an abundance 

 of food. Thus, in Arcella polypora the progenitors of twenty-six 

 families ranged in diameter from 23 to 35 units of 4.3 /x each, with 

 a mean diameter of 30.42 units, whereas their first offspring 

 ranged in diameter from 21 to 32 units with a mean diameter of 

 27.50 units. No definite reason can be given for this decrease in 

 size under laboratory conditions, but perhaps the abundant food 

 supply is responsible, resulting in the initiation of fission before 

 the cytoplasm has increased to the amount normally present 

 when the animals are in their natural habitat. 



The results of these experiments prove that size and spine 

 number in Arcella dentata are affected by the food supply. 

 Selection experiments involving these characteristics in this 

 organism and probably in other similar organisms, as well, must 

 therefore be carried out so as to provide a constant food supply. 



This factor, however, was carefully controlled in the selection 

 experiments that have been reported on Difilugia (Jennings, '16), 

 Centropyxis (Root, '18), and Arcella (Hegner, T9 a). 



THE ADDITION OF SODIUM SILICATE 



The fact that Whitney ('16) caused the transformation of the 

 rotifer Brachionus pala into the variation Brachionus amphiceros 

 by the simple addition of sodium silicate to the medium in which 

 they were reared, led to the use of this substance in experiments 

 on Arcella dentata. It seemed probable that the presence of an 



