THE THYROID AND PARATHYROIDS 315 



undergo metamorphosis a few days after the application of 

 the thyroid and long before the control animals do so. The 

 reaction was supposed to be a delicate test for the presence of 

 physiologically active thyroid substance. 



The reaction however does not appear to be a specific reaction 

 of thyroid substance since it has been found to occur after 

 administration of iodides and iodized blood-serum. Kendall 

 states that the action on metamorphosis (which occurs with 

 small doses of his ' ' thyroxin " ) is not due to the organic 

 nucleus, but is due to the iodine in the molecule, which breaks 

 off as hypoiodous acid (HI 0). 



Carrel states that tissues grown in vitro increase several 

 times as rapidly in the presence of thyroid substance as in its 

 absence. 



The administration of thyroid substance and of iodide to 

 animals has yielded some interesting results. In determining 

 the effect on gross weight most of the earlier observations were 

 made on adult animals. In regard to the effect on growth 

 in young animals, some observers have noted an increase in 

 the rate of growth, others a decrease, while still others have 

 recorded that no distinct effect was produced. Cameron was 

 the first to use a dose based rigidly on and bearing a constant 

 ratio to the body weight of the animal at the time of adminis- 

 tration. He has found, as the result of a series of very careful 

 experiments, that continued small doses of desiccated thyroid 

 gland administered to young white rats produce (a) a definite 

 and invariable decrease in rate of growth ; (b.) hypertrophy of 

 the organs concerned with increased metabolism heart, liver, 

 kidneys, adrenals, etc. (confirmatory of Hoskins and Herring) ; 

 (c) disappearance of fat (confirmatory of Hoskins and Herring). 

 The decrease in rate of growth is proportional to (a ) the amount 

 of thyroid administered and (b) the iodine content of the gland 

 given. The hypertrophy varies with the dose and length of 

 time during which it is administered, and appears also to be 

 proportional to the iodine content of the dose. Sodium iodide 

 given in quantities varying from amounts equal in iodine 

 content to the thyroid doses to amounts a hundred times as 

 great produces no effect on growth rate and no hypertrophy. 

 The effects produced are not due to protein feeding, autolytic 

 products, or any similar .cause, but specifically to thyroid 

 tissue, or some constituent of it. Both thyroid and iodide 



