HORMONE FACTORS IN GROWTH AND DEVELOPMENT 187 



Relation of lodin to Developmental Processes. Swingle (&) (c) (d) 

 (e) has discovered that in tadpoles inorganic iodin has the same effect of 

 bringing about precocious metamorphosis as has thyroid substance. In fact, 

 Swingle (6), and Allen (;') showed that even in thyroidectomized tadpoles 

 iodin can bring about metamorphosis. Swingle, therefore, claims that the 

 iodin is the effective principle in the thyroid hormone. There are, however, 

 several reasons that make this conclusion doubtful. It is possible that in 

 the amphibians the centralization of the thyroid function is not yet so far 

 carried through as in mammalians and that in the amphibian organism 

 perhaps all cells possess the ability of elaborating the thyroid hormone if 

 they are supplied with enough iodin. Or it may mean that there is, in 

 the amphibian body, in addition to the thyroid another organ that may 

 be able, in the absence of this gland, to produce a compensating (or 

 identical) hormone. In favor of this view are the experiments made 

 by Hoskins (c) which show that, in complete absence of the thyroid, feed- 

 ing of hypophysis is able at least to initiate and in some cases even to com- 

 plete metamorphosis. Allen's experiments, indeed, seem to show that in 

 amphibians the functions of thyroid and hypophysis are very much alike; 

 the differences between these two organs having not reached the high 

 degree as they present in mammals. It is furthermore possible that in 

 the amphibian organism the metabolic mechanisms have not yet been so 

 far specialized as in the mammalians and that consequently these mechan- 

 isms are able to react not only to the thyroid hormone but to many other 

 substances. In favor of this would be some results recently reported by 

 Laura Kaufman (6), according to whom other substances known to in- 

 crease metabolism, such as salicylic acid, are able to cause metamorphosis 

 of the axolotl ; before drawing further conclusions, however, it seems neces- 

 sary to repeat Kaufmann's experiments. That the effect of the iodin in 

 tadpoles must be due to some peculiar mechanism in these animals and 

 may, therefore, not permit of any general conclusions regarding the role 

 of iodin in the thyroid hormone is evident from the entirely different 

 behavior of closely related animals. Uhlenhuth in as yet unpublished 

 experiments has repeated Swingle's experiments on the effect of inorganic 

 iodin on the development of tadpoles and has found that, as reported by 

 Swingle, small quantities of iodin (2 to 3 drops of 1/20 m. solution in 

 1000 c.c. water) are able to hasten growth of the legs, development of 

 the mouth and gut; the tadpoles are extremely sensitive to iodin and 

 slightly larger quantities of iodin (4 to 5 drops) result in death. Re- 

 garding the quantitative effect the iodin is doubtlessly inferior to the 

 iodothyrin, since the latter will bring about metamorphosis far more 

 quickly than iodin. Uhlenhuth subjected salamanders also to the action 

 of iodin compounds, but with negative results. Recently Uhlenhuth has 

 repeated his former experiments on a larger scale and obtained the fol- 

 lowing (as yet unpublished) results. Salamanders are less susceptible 



