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 15000 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: — 
WATER LecitHin 
June 12 Irmm. 18 mm. 
June 21 12 mm. 18 mm. 
July 18 13 mm. 2I mm. 
Aug. 5 15 mm. 27 mm. 
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. Ido 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. 
Effects 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 
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