260 



Experimental Zoology 



by a sugar solution to that of normal sea water. If the increase 

 in length still occurs, the results must be ascribed to the chemical 

 effects of the salts rather than to the osmotic pressure. 



Effects of Heat on Growth 



Of the external factors that influence the rapidity of growth, 

 temperature has long been known to have the most influence ; 

 and since heat facilitates chemical reactions, it is generally as- 

 sumed that it acts in the same way in the organism. 



Higgenbotham kept frogs' eggs at 60, 56, 53, 51 F., and 

 found that they hatched respectively in 9, 14, 20, and 20 days, 

 and changed into frogs in 73, 161, 171, and 235 days. 



The rate for tadpoles has been measured by Lillie and Knowl- 

 ton (1898) and by O. Hertwig (1898). For the tadpoles of Rana 

 mrescens, and of the toad, Bufo lentiginosus, Lillie and Knowl- 

 ton obtained the following results : 



The rate of growth for the frog increases up to nearly 30 C. and 

 then decreases. The upper limit for the toad is about the 

 same or a little higher. 



Hertwig has compared the rate of growth of tadpoles of 

 Rana fusca at different temperatures. Their maximum nor- 



