498 J. A. DAWSON 



did not become a factor influencing the divison rate until after 

 the first five days. During the second and third five-day period 

 the divison rate of subculture 3 is distinctly lower than that of 

 subculture 4. Here, no doubt, the increasing excretion products 

 had a depressing effect. 



In each line of subculture 4 the animals seemed at all times 

 perfectly typical. In mass culture of subculture 3, however, 

 on December 26th, two animals were present on the slide when 

 it was examined. One, as usual, was removed. On the 27th, 

 upon examination it was found that this animal had divided to 

 give two and that these were united peristome to peristome. 

 On the 29th they were more firmly joined and died that night. 

 In this case, since all animals but one had been removed daily it 

 would hardly seem that lack of food had brought on this ' mis- 

 cible' state, since animals in set 1 were much more numerous 

 on the depression slides and were still dividing, though slowly, 

 but rather that the excretion products of the animals themselves 

 were the underlying factor. 



This experiment indicates clearly that cannibalism does not 

 take place while the culture medium is comparatively fresh and 

 also that the greatest amount of cannibalism does not occur in 

 a medium in which the food supply is much depleted, but in one 

 in which the scarcity of food is just beginning to be felt. Thus 

 it seems probable that the accumulation of excretion products 

 plays a part in inciting cannibalism. Jennings ('10) stated 

 that the cause of conjugation was "a decline in the nutritive 

 conditions after a period of exceptional richness that has in- 

 duced rapid growth and multiplication." These conditions have 

 in this case been duplicated with the result that instead of 

 conjugation cannibalism occurred. 



4. The physiological effect of cannibalism 



On December 10, 1917, three apparently typical animals were 

 isolated from three separate stock cultures coming from the 

 original culture A. On December 11th, each animal had divided 

 twice, giving four animals, and these were isolated to form the 



