236 W. W. SWINGLE. 



larvae were fed equal quantities of dibromtyrosine crystals, whereas 

 the remaining twenty animals received no food of any kind 

 and served as controls. Such a culture was considered necessary 

 in order to note any effects starvation might bring about on the 

 progress of metamorphosis since the animals reared on the tyro- 

 sine compounds were given very little food. 



It should be pointed out here that the animals in the dibrom- 

 tyrosine solutions received considerably more bromine than the 

 animals of the iodotyrosine cultures received iodine, despite the 

 fact that equal quantities of the two substances were fed to 

 each larva. This is obvious enough since the atomic weight of 

 iodine is 126.92, while that of bromine is but 79.92. Conse- 

 quently if equal amounts of the two tyrosine compounds were 

 fed or put into solution in given amounts of water as was the 

 case in the present experiment, the number of bromine atoms 

 per milligram of dibromtyrosine would be nearly double the 

 number of iodine atoms per milligram of diiodotyrosine. In all 

 the experiments recorded in this paper this fact has been ignored 

 and the two tyrosine compounds have been administered in 

 equal amounts as though they were chemically equivalent. The 

 experiment began June 22, 1923. On this date none of the 

 small immature larvae revealed the slightest indication of meta- 

 morphosis. June 28 all animals of the iodotyrosine-fed culture 

 showed marked reduction of the gills three animals had only 

 the stumps remaining. The tail-fin was undergoing reduction. 

 Sand was placed in each finger bowl in order that the animals 

 might crawl out of the water as metamorphosis progressed ; this 

 is a necessary precaution, otherwise the animals will drown over 

 night. 



Examination of the larvae reared in the dibromtyrosine solu- 

 tion showed no change; this was also true of the fed and unfed 

 control cultures. 



July 4, twelve days from the beginning of the experiment, 

 all of the diiodotyrosine-fed larvae had completely transformed 

 and left the water. Their gills and tail-fin had disappeared; 

 the branchial clefts had closed and the larvae had shed their 

 skins but the pigmentation characteristic of the adults of this 

 species had not appeared. Fig. I, A, is a photograph of four 

 iodotyrosine-fed animals. Compare this with Fig. I, B, which 



