246 W. W. SWINGLE. 



normal tadpoles with intact thyroids uncontrolled by thyroidless 

 animals might well have concluded from the results of such an 

 experiment that pituitary tissue exerts a direct stimulus to meta- 

 morphosis and is thus equivalent to the thyroid. 



Recently ('22) Romeis claims to have isolated an absolutely 

 iodine-free substance from the thyroid gland which when admin- 

 istered to frog tadpoles exhibits all of the physiologic effects of 

 thyroid gland tissue. The writer is skeptical of the validity of 

 this claim and for several reasons: (i) All active substances so 

 far isolated from the thyroid contain large amounts of iodine. 

 Thyroxin the active principle contains sixty-five per cent, of 

 iodine as an integral part of the molecule; and it is known that 

 thyreoglobulin containing no iodine is physiologically inert; 



(2) Experiments on tadpoles have shown (Rogoff, '18-19) 

 that blood coming from hyperplastic thyroid glands with ex- 

 tremely low iodine content fails to induce tadpole metamorphosis; 



(3) To date the only substances known to metamorphose thy- 

 roidless tadpoles are thyroid or iodine in some form presumably 

 in the last analysis organically combined. While writing this 

 paper the writer came across a second communication from 

 Romeis which practically amounts to a retraction of his earlier 

 claims. He found that iodothyrine and iodothyroglobulin will 

 induce the metamorphosis of tadpoles in dilutions of one in a 

 million; thyroxin produced the same effects on growth and meta- 

 morphosis in dilutions of one in ten million consequently says 

 Romeis: These figures suggest that effects (on tadpoles) from 

 so-called iodine-free materials from the thyroid may have been 

 contaminated with minute amounts of thyroxin. 



There can be little doubt that this is the real explanation of the 

 results obtained by this investigator with so-called iodine-free 

 substances from the thyroid. 



There seems to exist a fundamental difference between mam- 

 mals and amphibians in regard to their physiologic response to 

 thyroid and iodine administration. This difference is not suffi- 

 ciently understood by those who attempt to compare these two 

 verebrate groups. The following experiment of Kendall ('19) 

 is an excellent illustration of the point: He found that injections 

 of pure thyroxin into mammals is followed by a very definite and 

 marked physiologic response. But when the hydrogen of the 



