38 



Scientific Proceedings (87). 



cule, etc., the worm-fed animals actually received less food than 

 the thymus-fed animals. Therefore, the thymus-fed series grew 

 more rapidly than the worm-fed series. Had we not recorded the 

 number of pieces which were fed to each series, we would have 

 been led to the erroneous conclusion that thymus promotes growth 

 in a specific way. 



That thymus does not promote growth in a specific way is also 

 seen from the two following curves obtained from two series 

 (A, B, 1917) of A. opacum. Again in both series the food was 

 given in pieces on a forceps; but to each animal food was given 

 every day, until it would take no more food. Here the worm- 

 fed animals grew even more rapidly than the thymus-fed series. 



When the differences in the quantity of food become still 

 greater, the animals show differences in size which are far greater 

 than any reported due to thymus feeding. This is demonstrated 

 by three curves obtained from the average sizes of three series of 

 A. opacum (C, D, E, 1917). 



These experiments show clearly that the quantity of food had 

 a greater influence upon the growth of the salamander larvae than 

 the quality of food; amphibians in general react more promptly 

 to small quantitative food differences than warm-blooded animals. 

 Extensive experience with amphibians shows that out of a set of 

 larvae, certain individuals may take an exceedingly small amount 

 of food, while in other sets, several animals take surprisingly large 

 quantities of food from the very beginning. Differences in size 

 like those produced artificially in the above series may, then, 

 arise by spontaneous and uncontrolled action of the animals 

 themselves and create the impression that a certain kind of food 

 used in a certain experiment caused the difference. 



2. Concerning development and metamorphosis, different 

 species of even the same genus seem to react quite differently to 

 the thymus. 



In A. tigrinum, the thymus feeding produced neither accelera- 

 tion nor retardation of metamorphosis, if we do not include, for 

 the present, one animal to be mentioned later on. This is indi- 

 cated in the curves R, S, T, U, 191 7, and in six pictures. 



In A. opacum the thymus diet caused a decided acceleration 

 of the development of the gills and the skin (demonstrated by 



