6 THE ACTION OF THE LIVING CELL 



tion of Carrel, that a change of the medium surround- 

 ing the tissue is not sufficient to keep the culture in a 

 healthy condition. However, he found that if the tis- 

 sue is carefully washed and then supplied with fresh 

 culture media a normal rate of growth ensues. Evi- 

 dently, by such a process autotoxic substances are re- 

 moved. 



Many workers have observed that certain infusoria 

 when left undisturbed do not remain scattered through 

 their aqueous environment, but aggregate into more 

 or less dense groups. Having observed that Para- 

 moecia may aggregate about a drop of dilute acid or 

 carbon dioxide solution, Jennings (1902) concluded 

 that the carbon dioxide produced by the infusoria 

 themselves was responsible for their aggregation in 

 a culture medium. This may or may not be true, but 

 at any rate it appears that some product produced by 

 the cells themselves is responsible for the observed 

 effect. 



Let us now consider the effect of one type of cell 

 upon the growth of another. Here two possibilities 

 exist, — that of symbiosis, and that of antibiosis, numer- 

 ous examples of each being recorded in the literature. 

 For example, Holman (1923) found that in the pres- 

 ence of a minute anaerobe resembling B. pneiimo- 

 sintes,B. influenzae exhibits a good growth upon blood 

 agar slants, whereas in the absence of the former little 

 or no growth of the latter takes place. The opposite 

 effect is recorded by Passini (1926), who observed 

 that B.putrificiis verrucosus is able to destroy 5. tuber- 

 culosis when present in the same culture. 



That one type of cell may profoundly affect the 

 behavior of another type is shown by the remarkable 



