320 THE EFFECT OF IODINE AND lODOTHYRIN. IV 



Of three series of larv£e of Amhy stoma maculatum from the same 

 brood, Series 1 was fed earthworms and kept in iodine-free water, 

 Series 2 was fed thymus and kept in iodine-free water, Series 3 was 

 fed thymus and kept in water to which, from the 26th day on, 1 drop 

 of a 1/20 M solution of inorganic iodine per 1000 cc. of water had been 

 added. 



Growth began to decrease in both thymus-fed series at an age of 

 19 days and practically ceased at an age of 26 days. From this time 

 on inorganic iodine was administered to Series 3 as stated above, 

 but did not improve the growth of this series at all. At the 54th 

 day both thymus-fed series were divided into two lots and, in order 

 to make sure that the ineffectiveness of the iodine was not due to a 

 loss of the ability to grow, earthworms were fed instead of thymus. 

 The result was as follows: In Series 2 the larger and stronger larvae 

 were continued on thymus; they did not grow, and finally died at an 

 age of 82 days. The smaller larvae received earthworms instead of 

 thymus; they immediately began to grow, reached a normal size and 

 finally metamorphosed. In Series 3 to which the iodine was adminis- 

 tered the smaller larvae received earthworms instead of thymus; this 

 change of the diet again resulted in vigorous growth and in meta- 

 morphosis. The larger larvae were continued on thymus, but in spite 

 of the administration of iodine did not grow at all, until, at the 82nd, 

 day, earthworms were used as food instead of thymus. This diet 

 again resulted in normal growth and metamorphosis. 



The objection could be raised that the amount of iodine was too 

 small to permit of growth and metamorphosis. It is possible that the 

 concentration of iodine was not high enough to permit of normal 

 growth and metamorphosis. If, however, a lack of iodine had any- 

 thing to do with the inhibition of growth and metamorphosis, even 

 the slightest amount of iodine should have produced an improvement. 

 Since even no improvement could be obtained by iodine-administra- 

 tion, but immediately followed the administration of earthworms, the 

 conclusion seems to be justified that the inhibition of growth and 

 metamorphosis of thymus-fed salamander larvae is not caused by a 

 deficiency of the thymus in iodine, but by a deficiency in certain 

 substances, as yet unknown, which are contained in the earthworm. 



