238 W. W. SWINGLE. 



shows four di-bromtyrosine-fed larvae. The animals in Fig. I, B, 

 were photographed twenty-five days after the experiment began, 

 hence had been on the dibromtyrosine diet twice as long as the 

 animals of Fig. I, A, had been fed iodotyrosine. The meta- 

 morphosed animals were very weak and died w r ithin three to 

 five days after metamorphosis. During the twelve days the 

 experiment continued, they were each fed three small worms, 

 the animals of the other cultures (except one) received the same. 



The dibromtyrosine culture was continued twenty-six days 

 after transformation of the iodotyrosine culture had occurred, 

 i.e., thirty-eight days from the date of first feeding. At the end 

 of this time eleven animals metamorphosed, the remaining larvae 

 did not transform. The culture was abandoned July 31. The 

 fed and unfed control groups were also given up at this time. 



It is an interesting fact that none of the control animals of 

 either starved or fed cultures metamorphosed during the thirty- 

 eight days of the experiment, whereas eleven larvae of the di- 

 bromtyrosine culture did transform. The experiment indicates 

 that unfed salamander larvae can be forced to metamorphose if 

 reared in very strong solutions of dibromtyrosine over long 

 periods. The bromine in the tyrosine molecule is thus seen to be 

 not entirely inert as regards metamorphosis though it cannot be 

 compared with iodine. For instance, diiodotyrosine solutions of 

 about one-half the concentration of the dibromtyrosine, meta- 

 morphosed salamander larvae of equal size and developmental 

 stage in a period ranging from seven to twelve days, whereas the 

 much stronger bromtyrosine solutions caused about two-thirds 

 of the larvae of the culture to transform between the thirtieth 

 and thirty-ninth day. 



In an earlier paper ('22) the writer called attention to the fact 

 that the hind legs of thyroidless and pituitaryless tadpoles reared 

 in strong dibromtyrosine solutions grow larger than the limbs of 

 like animals on an algae diet but that such animals do not meta- 

 morphose. Here again the evidence indicates that the bromine 

 ion is not entirely passive since it does stimulate to a slight degree 

 the growth of the tadpole's hind limbs. But the degree of activity 

 of the bromine in the tyrosine molecule is not comparable to that 

 of iodine. The following experiment clearly shows the great 

 difference in activity between the two substances. Three sets of 



