I04 ANIMAL AGGREGATIONS 



organisms on filter paper or on agar than when they are grown free 

 in hquid broth; and when the organisms are repeatedly filtered off so 

 that the physical effects of crowding are periodically eliminated, the 

 growth period is not thereby prolonged. There can be no doubt but 

 that the exhaustion of food materials does play an important role 

 in the limitation of cultures, but the question as to how important 

 this is in comparison with the accumulation of waste products or 

 "autotoxins" has not been decided. 



Henrici says, "The idea that growth is limited by the accumula- 

 tion of some toxic substance is the one that seems to be most general- 

 ly accepted, though the evidence for it is far from being convincing." 

 The evidence supporting the idea that the toxic substances are im- 

 portant is as follows: Eijkman (1904, 1906, 1907), working with a 

 number of species of bacteria, grew them in gelatine until the culture 

 was densely crowded. He found then that if he took a part of this, 

 heated it to boiling and, after cooling, reinoculated it, it would then 

 support growth; but that another part, heated only sKghtly and then 

 allowed to resolidify, would not produce growth in a new surface 

 inoculation. Since heating to boiling-point would add no new food 

 material, Eijkman concluded that he was dealing with some ther- 

 molabile product of metabolism or a more specific growth-inhibiting 

 substance. 



Further experiments showed that the toxic material would not 

 pass through a porcelain filter, that heating which killed the living 

 organisms destroyed the toxicity of the medium, and that treatment 

 with such volatile agents as ether and ammonium sulphide not only 

 killed the bacteria but rendered the medium again capable of sup- 

 porting growth after the volatile material was driven off. They also 

 showed that if gelatine in which Bacillus coli had grown was resolidi- 

 fied into a plate and reinoculated with more of the same organisms 

 and covered with a layer of fresh gelatine, there would be no growth; 

 or, if fresh gelatine was inoculated with B. coli and then had one part 

 covered with fresh gelatine and another with the so-called "coH- 

 gelatine," growth would take place in the former only. Inoculation 

 of paper dipped in agar and then placed over the coli-gelatine did not 

 yield a growth unless the coli-gelatine had first been heated. 



