34 SYMBIONTICISM AND THE ORIGIN OF SPECIES 



growth observed was dependent upon food materials present 

 in the planted liver tissue. When this material is exhausted, 

 growth necessarily ceases. The pertinence of this suggestion 

 was strengthened by the observation that the extent of 

 cloudiness about the planted tissue bore a direct relation- 

 ship to the size of the tissue fragment explanted. The 

 problem then resolved itself into discovering what food 

 material might be deficient in the culture media. It ap- 

 peared likely that it was some nitrogenous foodstuff. Vari- 

 ous sugars and salts had been tested with practically no 

 indication of variations in the results. Nitrogenous food 

 had been supplied to the media from two sources: the 

 infused Hver and peptone; but it appeared probable that the 

 nitrogen from these sources was not in a form available for 

 mitochondrial utihzation. This naturally led to an analysis 

 of the possible sources of nitrogen for mitochondria in the 

 living cell. Obviously, nitrogen compounds in the Hving 

 cell have two sources of origin: those brought to the cell 

 in the form of amino acids by the blood, and the waste 

 products of nitrogenous metabohsm, chiefly in the form of 

 urea. On the basis of these speculations, a new series of 

 culture media was devised in which various amino acids 

 and urea were used. These chemicals were added to the 

 rabbit hver infusion medium in varying dilutions. 



The prehminary experiments consisted in testing the 

 various amino-acid and urea media. Of the amino acids, 

 only tryptophane appeared to satisfy the conditions neces- 

 sary for free surface growth of mitochondria. It is possible 

 that certain combinations of other amino acids might have 

 supported growth; a few were tested with negative results. 

 The urea medium also satisfied the conditions for inde- 

 pendent mitochondrial growth. In general, it appeared 

 that growth was more luxurient in the urea media than in 

 that containing tryptophane. 



Attention was again turned to the deep clouded medium 



