' W. W, SWINGLE 601 



of the animals in each of the cultures. There was no continuous up- 

 ward or downward growth curve depending upon the iodine concen- 

 tration. Thus, the animals of Culture 2 outgrew both controls and 

 the animals in stronger concentrations; the animals in Culture 3 

 were smaller than those of Cultures 2 or 4. Strong concentrations 

 of iodine bring about cessation of growth very quickly, and differ- 

 entiation is stimulated, whereas very weak solutions Such as those of 

 Cultures 1 and 2 tend to stimulate both growth and differentiation. 

 Rapid differentiation apparently does not occur along with rapid 

 growth; an overdose of thyroid extract or iodine to frog larvae imme- 

 diately starts the mechanism of differentiation. If the larvae are very 

 young, not only does growth cease, but differentiation does not pro- 

 ceed very far before death of the organism supervenes due to the 

 extreme height to which metabolic activities rise. The organism 

 literally withers away. 



The work of Janney is of interest in this connection.^ He has shown 

 that an overdose of the thyroid hormone leads to a loss in weight and 

 size of the organism ; conversely, a certain minimum dose results in a 

 gain in both size and weight. This is very well shown in the present 

 experiment, the animals of Cultures 1 and 2, receiving minimum 

 doses of iodine, outgrew the controls and the animals of the stronger 

 concentrations. As shown elsewhere^-^ by the writer, overdosage 

 with iodine or thyroid extract leads to decrease in size and great 

 emaciation of the animals. The conclusion in regard to growth is 

 that iodine in minimum doses stimulates growth, though not to any 

 great degree, and in larger doses it leads to cessation of growth and 

 tissue disintegration or absorption. Iodine feeding has but little 

 effect on pigmentation. If fed in sufficiently strong concentrations 

 the animals appear darker than the controls. 



Experiment Z. Bromine Feeding. — The results obtained by feed- 

 ing iodine did not, however, rule out the possibility that the accel- 



■^ Janney, N. W., III. Studies in thyroid therapy. The effect of the thyroid 

 hormone as determined by a clinical metabohc and dietetic investigation. New 

 points of view on thyroid function in health and disease, Arch. Int. Med., 1918, 

 xxii, 187. 



^Swingle, W. W., The acceleration of metamorphosis in frog larvae by thy- 

 roid feeding, and the effects upon the alimentary tract and sex glands, /. Exp. 

 ZooL, 1917-18, xxiv, 521. 



