• W. W. SWINGLE 603 



During the latter part of the experiment three and four times as 

 much bromine (chemically equivalent to the iodine) was fed as was 

 given in the highest concentrated iodine culture, but the only effects 

 produced on the larvae were atrophy of the tail and slight emaciation. 

 Proper precautionary measures were taken that the bromine did 

 not pass out of solution as fumes, thus weakening the concentration. 

 It may be added that this experiment was tried out on two species of 

 Anura, Rana sylvatica and Rana catesbiana, with negative results in 

 each case. 



Discussion of Bromine Feeding. 



This experiment though negative has considerable theoretical in- 

 terest in its bearing upon the relation of iodine to the thyroid. Ken- 

 dall,^ who claims to have isolated and synthesized the physiologically 

 active principle of the thyroid, lays especial emphasis upon the pres- 

 ence of the oxy-indol nucleus of his compound, thyroxin, and con- 

 siders the presence of iodine in the combination of little importance 

 as it acts probably as a catalyst and in no other way. He says in this 

 regard: "The presence of iodine in the compound must exert some 

 influence, and this influence is probably that of rendering the active 

 groups (CO — NH) more reactive." He thinks that if the iodine were 

 absent from his thyroxin, it would take merely a greater "working 

 pressure" to bring about the reaction of the thyroxin. "The sub- 

 stitution of iodine by hydrogen or chlorine or bromine would un- 

 doubtedly be followed by an alteration in the degree of reactivity of 

 the substance, but its gross chemical nature and properties would not 

 be altered thereby. That iodine breaks off from the molecule and is 

 used as iodine per se for any purpose seems absolutely impossible." 



Dr. Kendall's work deserves careful consideration. However, the 

 writer finds it impossible, in the light of the bromine experiment just 

 recorded, to believe that this halogen can substitute in any way for 

 iodine in its physiological activity. Split off the iodine molecule 

 from the thyroxin complex isolated by Kendall and probably the 

 residue would be a physiologically inert substance. 



If the gross chemical nature and properties of thyroxin would 

 not be altered by splitting off the iodine molecule, how is it that 

 thyroidless tadpoles, which never undergo metamorphosis under 

 normal conditions, nevertheless quickly transform into frogs if fed 



