i66 ANIMAL AGGREGATIONS 



ture medium. Later, after the evidence furnished by Cutler and 

 Crump showed that the growth of Colpidium is much more facih- 

 tated by the "contaminating bacteria" which they found ordinarily 

 associated with this infusorian than it is by Sarcina, which they 

 employed as a food organism, Robertson (1927) considered the pos- 

 sibility that Colpidium and other ciliates are restricted in their food 

 to certain species of bacteria for which they modify the nutrients 

 available, thus existing in a sort of symbiosis with their food species. 

 However, he concludes that whether the allelocatalytic effect origi- 

 nates with the ciliates or with associated food organisms, it remains a 

 mutually accelerative effect of contiguous organisms upon their 

 reproductive rates. 



Robertson shows that his results are not due to the carrying-over 

 of twice the amount of parent culture medium when 2 organisms, in 

 place of I, are transplanted; for in his experiments infusorians 

 washed with medium similar to that into which they are subse- 

 quently subcultured show the effect even better than unwashed ani- 

 mals. Neither is the effect due to mere numbers, for recently divided 

 or dividing individuals, if transferred together, act as single animals. 



To explain the observed results, Robertson advances the following 

 hypothesis (1923). During nuclear division each nucleus retains the 

 charge of autocatalyst with which it was provided, and adds to it 

 during the course of nuclear synthesis. At each division the auto- 

 catalyst is shared between the nuclear substance and the surround- 

 ing medium in a proportion determined by its relative solubility and 

 by its affinity for chemical substances within the nucleus. The mu- 

 tually accelerative or allelocatalytic effect of contiguous cells is due 

 to each cell's losing less of the autocatalyst to the medium because of 

 the presence of the other. According to this hypothesis, the auto- 

 catalytic effect of growing colonies, whether attached or detached, 

 is due to the same cause. 



Fischer's work (1923), in which he found that fibroblasts grow in 

 vitro only when tissue cells are numerous and close together, can be in- 

 terpreted as giving supporting evidence to the allelocatalytic hypoth- 

 esis. Burrows (1924), growing cancer cells in vitro, found a similar 

 stimulation; this and other similar evidence from tissue culture has 



