HOEMONE FACTORS IN GEOWTH AND DEVELOPMENT 183 



increases metabolism is well known (Magnus-Levy, Schb'ndorff, Mayerle 

 and numerous more recent investigators) and Kendall, who succeeded 

 in isolating the probably pure thyroid hormone (thyroxin) and deter- 

 mined its chemical constitution, possibly found an explanation why this 

 substance should so efficiently raise the metabolism. According to Ken- 

 dall (c) 1/3 mg. of thyroxin increases the basal metabolism of an indi- 

 vidual of approximately 150 pounds by 1 per cent. Since Baumann 

 discovered that the active principle of the thyroid contains iodin, much 

 has been speculated about the role of this iodin. Kendall (c) thinks that 

 the presence of iodin in the thyroxin merely renders the active groups 

 of this compound more reactive, but that it is not used as such in the 

 metabolic processes. In accordance with this opinion is the fact that the 

 more iodin the thyroid substance employed in the experiment contains 

 (Lenhart, Graham), the more effective is thyroid feeding in bringing 

 about certain changes in tadpoles presently to be discussed, while most 

 iodin compounds other than those contained in the thyroid, and espe- 

 cially the inorganic, have no effect (Morse) on development. Also as 

 regards the physiological effects certain organic iodin compounds, such 

 as gorgonin and spongin, though they contain far more iodin than does 

 the iodothyroglobulin prepared from the thyroid, are less effective than 

 the latter substance (Oswald). Eecently it has been demonstrated, how- 

 ever, that at least some organisms are able to utilize the inorganic iodin 

 in the morphogenetic processes without the intermediation of a thyroid 

 gland (Swingle (&)). We shall later revert to these experiments. So far 

 as the morphogenetic effect of the thyroid hormone is concerned, the 

 most important question, it seems to me, is that as to the changes which it 

 effects in the metabolism. Does the thyroxin actually, as Kendall claims, 

 merely increase the reactions going on also without it in the organism, 

 without changing or adding any reactions and must we think that a mere 

 quantitative change is able to produce all that multitude of new forms 

 and structures that we see arise in a thyroidless organism after the admin- 

 istration of the thyroid hormone ? 



Relations in Mammals. The effects of the thyroid hormone upon 

 the development of the mammals, as far as they have been studied, are 

 probably well known to most readers of this handbook and will be dis- 

 cussed here only briefly. They can be studied in various ways. Important 

 information was obtained from the study of human subjects who in pre- 

 puberal periods of life developed hypo- or a-thyroidism (for literature see 

 Falta (a)). The most conspicuous consequences of thyroid deficiency are 

 the nearly complete cessation of growth, a lack of proper ossification of the 

 skeleton, in consequence of which the epiphyses of the limb bones remain 

 separated, disturbances in the development of the teeth, retention of an 

 infantile condition of the sex organs and failure of the brain to develop 

 beyond an infantile condition. Administration of thyroid preparations 



