300 THE EFFECT OF FOOD 



whilst a much slighter negative effect (averaging 1.4 

 per cent.) was produced in only three. The relative 

 variation in size produced by 100 grams of animal foul- 

 ing 1 litre of water for 1 hour is also given. These 

 values are somewhat more variable than those repre- 

 senting the absolute variation, but they to some extent 

 corresponded to the amount of nitrogenous matter 

 actually excreted into the water, as was proved by 

 chemical analysis of the various samples. 



We may conclude, therefore, that under certain con- 

 ditions products of metabolism may stimulate an organ- 

 ism to increased growth, whilst under certain others 

 they may retard growth. What is the nature of these 

 excretory products which exert so potent an effect? 

 Observations made on the influence of various simple 

 substances on larval growth seem to throw some light 

 on the question. The results obtained with uric acid 

 and urea are given in the accompanying table: 



Here we see that uric acid in moderate amounts 

 exerts a very favourable influence on the size of larvae. 

 It is only when the proportion is raised to 1 in 28,000 

 (a more than half saturated solution), that an unfav- 



