External Factors that Influence Growth 257 



It has also been shown for other molds, Aspergillus, Penicillium, 

 and Botrytis, that alkaloids and other poisonous substances if 

 present in small amounts accelerate growth. 



Few cases of this sort can be given for animals. The best- 

 known case perhaps is that of lecithin. Danilewsky has found 

 that lecithin has a marked influence on the rate of growth. Tad- 

 poles were placed in a solution containing one part of lecithin 

 to 1 5000 parts of water. The rate of growth of the tadpoles was 

 compared with that of similar tadpoles kept in water alone. 

 The results are given in the next table : 



The lecithin tadpoles are three times heavier and nearly twice 

 as long as those in water alone. The results are ascribed to 

 the stimulating effects of the lecithin. I do not feel convinced 

 that this is the case, because any one who has reared tadpoles 

 in confinement must have found that great variations in size 

 are found in different dishes in which the conditions seemed to 

 be the same. Even in the same dish very great differences in 

 size are also found. 



Danilewsky claims to have obtained similar results with 

 young rabbits and dogs by injecting solutions of lecithin into 

 them at intervals. Desgrey and Zaky have also studied the in- 

 fluence of lecithin on growth, and reach the conclusion that the 

 beneficial effect is due to the stimulating effect of the substance. 

 Hatai also has found that in young rats lecithin causes an 

 increase in weight whether given as injections or with the food. 



Ejects of Salt on Growth 



The eggs of many animals, especially those laid on the land, 

 contain a sufficient quantity of materials to carry them through 



