BEGINNINGS OF CO-OPERATION 8l 



sible of an essential, growth-producing substance 

 with which it was provided, and adds to it dur- 

 ing the course of growth between divisions. At 

 each division, however, this substance is necessarily 

 shared with the surrounding medium in a propor- 

 tion that is determined by its relative solubility in 

 the culture water, and by its affinity for chemical 

 substances within the nucleus. The mutual speeding 

 of division by neighboring cells is due to each cell's 

 losing less of this necessary substance because of the 

 presence of the other. The more of this growth-pro- 

 moting substance there was in the cell, Robertson 

 thought, the faster would be the division rate; so 

 that any circumstance which would conserve the 

 limited supply would tend to speed up processes 

 leading to cell division. 



Stripped to essentials this hypothesis says that as 

 a result of the presence of a second organism both 

 lose less of an unknown something which is essen- 

 tial for division than would happen if but one were 

 present. Returning to the problem after the criti- 

 cisms of half a dozen years, Robertson affirmed that 

 all the data and conclusions on the subject that had 

 been issued from his laboratory remained valid save 

 that they might apply to the ^ associated food or- 

 ganisms and not to the protozoans themselves. 



Johnson has paid considerable attention to this 



