8 E. R. HOSKINS AND M. M. HOSKINS 



one month — a time much less than that usually required for 

 R. sylvatica kept in the laboratory. In the former series the 

 maximum time required for metamorphosis to begin was four 

 months, while in the latter series the corresponding time was 

 only seven weeks. 



The growth of our experimental and control animals seems 

 to have been different from Allen's ('18), who states that he 

 observed no difference between the controls and thyroidless 

 larvae until the time of metamorphosis. This point we studied 

 very carefully by taking hundreds of measurements of the grow- 

 ing animals. As shown in table 1, we found that in every one 

 of the experiments except one small series (5/5) in which the 

 thyroidless larvae all had defective jaws or muscles, the largest 

 thyroidless larvae grew more rapidly than the largest controls. 

 The same was true also of the average after elimination of those 

 suffering from inanition. For a few days after the operation 

 the larvae grew more slowly than the controls, but passed them 

 at an average size of 18 mm. (range 12 to 23 mm.). At this time 

 there were alive 260 operated and 218 control larvae, so the fact 

 of more rapid growth of the former seems well established. The 

 1918 series were not measured at this stage. This difference in 

 growth rate could not be accidental or due to the food supply, 

 because it was constant in every series from 1916 to 1918 

 larvae, the latter of which were especially well fed. By the 

 time of metamorphosis of the controls the largest corresponding 

 thyroidless larvae had attained the length of 58 mm,, and in 

 every series those thyroidless larvae without defects were all 

 larger than the controls when the latter had reached their maxi- 

 mum size. One control larva became 53 mm. long before meta- 

 morphosis and one reached the length of 50 mm., but with these 

 two exceptions none of the controls ever grew longer than 48 

 mm. before metamorphosis, the smallest being only 40 mm. long, 

 and the average 46 mm. before they became transformed into 

 frogs. 



In general, the 1917 larvae, which grew more slowly than the 

 1918 animals, became larger than the latter before metamor- 

 phosis, but the largest in 1918 were larger than the average of 



