io6 ANIMAL AGGREGATIONS 



ganism, Kuester (1908) finds that molds grown in a nutrient solution 

 produce conditions which check the growth of further inoculations 

 before the nutrient supply is exhausted. 



In the face of this evidence, Henrici is still unconvinced of the 

 general apphcabihty of the theory of the production of a toxic ma- 

 terial serving to limit growth, and holds, rather, to the idea that the 

 exhaustion of the nutrient material is the crucial point. He suggests 

 rightly that the Eijkman type of experiment may only show that 

 media may contain sufficient material to support a heavy population 

 without growth and still be able to support growth in a smaller 

 population; that heating the medium to kill the bacteria may cause 

 a release of nutrient material, making it available for the reinoculat- 

 ed organism; and finally cites the work of Graham-Smith (1920) in 

 which he was able to revive staphylococci by adding concentrated 

 meat extract and thus inducing new growth after the period of maxi- 

 mum growth had passed, and could postpone the death phase in- 

 definitely by small daily additions of meat extract. Henrici is prob- 

 ably correct in concluding that different factors may limit growth in 

 different cases and that there is no sound basis for believing in the 

 production of specific autotoxins. 



EVIDENCE FROM TISSUE CULTURE 



In tissue-culture work Carrel and Ebeling (1923) and Mottram 

 (1925) report a substance which inhibits growth of explants present 

 in extracts of aU adult tissues, in serum and even in extracts of em- 

 bryos. The latter have usually been found to favor growth in such 

 cultures. Heaton (1926) has found such a substance in yeast extracts 

 and in a number of adult-animal tissues, especially the liver. He 

 thinks that the failure of adult tissues to grow easily in vitro, and the 

 stoppage of growth of connective tissue in vivo, as contrasted with 

 the continued growth of epithelia, is to be attributed to this growth- 

 inhibiting substance. Heaton finds it to be thermostabile, though 

 destroyed by heating up to 125° C. It is soluble in water and alcohol 

 up to 75 per cent strength, but is insoluble in 97 per cent alcohol. It 

 seems to be destroyed by autolysis. Its action is greater on older 

 than on younger embryonic tissues. 



