594 IODINE AND THE THYROID. Ill 



stance into the general circulation under pressure of the organism's 

 needs. 



In the experiments recorded here, further evidence, indicating that 

 the conclusions stated are essentially correct, is presented. 



The problem, with which the present experiments are concerned, 

 w^as suggested by Professor E. Newton Harvey, in whose laboratory 

 the work was done, and to whom the writer is indebted for sugges- 

 tions, equipment, and criticism. Briefly stated, the problem was to 

 determine if the action of iodine upon Anuran metamorphosis is 

 specific, or if other closely related chemical substances, such as bro- 

 mine, exert a similar effect; to determine by quantitative feeding the 

 amount of iodine, or other substances if any were found, required to 

 produce metamorphic changes in tadpoles kept under uniform con- 

 ditions; and finally to compare the histological picture presented by 

 the thyroids of the iodine, bromine, and normally fed animals. 



Throughout the work, unless otherwise stated, the larvae used 

 were Rana sylvatica LeConte. All animals came from the same egg 

 mass, and hence were of the same age. They were permitted to de- 

 velop to a total length of 10 mm. before being used, though no food 

 was fed them during this interval, nor was any needed as the larvae 

 subsist and increase in size by utilization of the yolk in the body 

 cells. 



The food given consisted entirely of algae, practically all gathered 

 from the same pool, and Elodea from the University vivarium, fed in 

 equal quantities. The only animal matter the tadpoles received was 

 that entangled in, or living on, the algae. The environmental factors 

 of temperature, light, and water supply were kept as nearly uniform 

 as possible. Too much stress cannot be laid upon these factors in 

 any work concerned with growth and metamorphosis, as the extreme 

 variability often observed in the growth rates of frog cultures is due 

 largely, in the writer's experience, to slight disturbances of these 

 environmental factors. 



Each culture consisted of thirty- two larvae in 2,000 cc. of ordinary 

 tap-water, with fairly high calcium content. The water was changed 

 daily. 



Iodine and bromine were made up in chemically equivalent stock 

 solutions of m/10 concentration, the iodine in alcoholic, the bromine 

 in aqueous, solution. 



