184 E. UHLENHUTH 



in such persons (cretins) may bring about a renewal of growth and de- 

 velopment. In mammals the condition of athyroidism was produced by 

 removal of the thyroid gland soon after birth (Hofmeister, Eiselsberg (&), 

 Biedl (&) and numerous other investigators). The results are very similar 

 to those produced by a-thyroidism in man. Growth ceases at an early 

 date after the extirpation, the skeleton is incompletely ossified, the thymus 

 does not show the involution accompanying development of the normal 

 organism and the sex glands remain hypoplastic. Frequently the animals 

 die from emaciation (cachexia thyreopriva). 



Like all the processes of growth, regeneration, too, is greatly dimin- 

 ished after removal of the thyroid. This was shown by Marinesco and 

 Minea and by Walter, who cut the nerve after removal of the thyroid and 

 studied the process of regeneration. It is interesting to note that not only 

 regeneration is greatly delayed, but that even the degeneration of the cut 

 ends, which under normal conditions follows very quickly, proceeded at a 

 very slow rate, due to the retardation of all metabolic processes. 



Feeding thyroid gland to mammals results either in an increased or 

 decreased rate of growth according to whether a small or a large dose is 

 used. Lately E. R. Hoskins (a) has made careful observations on the 

 effects of feeding thyroid to rats and has given a full review of this subject. 

 Among the earlier observers Bircher found that one of the most con- 

 spicuous effects of feeding thyroid substance to mammals is the early 

 calcification of the long bones, a fact that corresponds to the opposite 

 phenomenon in thyroidless mammals. In general, however, the effect of 

 feeding thyroid to the normal organism is less conspicuous than the 

 changes that occur in cretins after administration of thyroid substance. It 

 is possible that the organism when in possession of a normal thyroid gland 

 is able to regulate in some way the amount of hormone circulating in the 

 blood, even if there be introduced an excess of it from outside. 



Relations in Amphibia. Far more conspicuous than in mammals are 

 the morphogenetic effects of thyroid feeding in amphibians. A large mass 

 of facts as regards the influence of the thyroid upon growth and develop- 

 ment in amphibians has accumulated since the publication of Guder- 

 natsch's famous experiments on the larvse of frogs and toads and, in fact, 

 the consequences of thyroid feeding have been studied far more completely 

 in amphibians than in any other group of animals. In 1912 Gudernatsch 

 (a) discovered that feeding thyroid gland to tadpoles hastens development 

 to such an extent that metamorphosis may take place within a few days 

 after the first administration. These experiments have been repeated by 

 many authors (Rornois, Kahn, Abderhaldeii, Allen, Swingle (a), E. E. 

 Hoskins) and always have given the same striking effects. Similarly, in 

 the caudate Batrachians such as the salamanders, feeding of thyroid or 

 keeping the larvae in a suspension of iodothyrin results in rapid trans- 



