HORMONE FACTOKS IN GROWTH AND DEVELOPMENT 185 



formation of the larvse into terrestrial salamanders (Laufberger, Uhlen- 

 huth (g), Jensen, Kaufman). In tadpoles the effect of the thyroid feeding 

 as described by Swingle (a) consists in an accelerated development of the 

 hind and fore limbs, in a rapid reduction of the tail, in the precocious dif- 

 ferentiation of the simple coiled intestinal tube into the stomach, small and 

 large intestines and rectum of the adult, in the precocious development of 

 the frog mouth and of the protrusion of the eyes, In salamanders the effect 

 of the thyroid feeding consists, so far as is visible, externally, in rapid 

 loss of the gills and of the fin of the tail and in shedding of the entire skin. 



In the earlier experiments (Gudernatsch) relatively large doses were 

 employed; this resulted, in spite of the accelerated development, in cessa- 

 tion of growth and even in diminution of size. Lenhard showed, how- 

 ever, that small doses when administered with an abundant supply of food, 

 especially carbohydrates, do not lead to diminution of size, but, on the 

 contrary, accelerate growth. The explanation would seem to be that if 

 large doses are administered, metabolism is increased to such an extent 

 that the animal cannot supply the excessive demand by the food intake 

 alone and draws on the tissues of its own body to build up the rapidly 

 differentiating new structures. 



In spite of the rapid progress of differentiation of nearly the entire 

 body there are, however, some organs that seem to remain unaffected by 

 the thyroid application. In the thyroid-fed tadpoles the sex glands do 

 not develop more rapidly than in normal larvse (Swingle (a) ). In salaman- 

 ders, metamorphosis of the thyroid-treated larvse may occur without de- 

 velopment of the pigmentation of the adult (Uhlenhuth (i), Jensen) and 

 of several toes that were not developed before administration of the thyroid 

 substance was begun (Uhlenhuth (t). It is, therefore, not unlikely that 

 the development of these organs is controlled by substances other than 

 the thyroid hormone, but which are produced by the action of the thyroid 

 hormone, and which act more slowly than the latter. 



Recently a method has been devised to extirpate the thyroid gland 

 from tadpoles at an early embryonic stage (Allen (,) (/) (g), Hoskins 

 (a) (&) ). The results of the extirpation experiments are in perfect accord 

 with those obtained by feeding the gland and were described by Allen, 

 Terry, Rogers and Hoskins. In the beginning, the larvse show normal 

 development ; but at the time when the hind limbs appear in the controls, 

 the thyroidectomized larvae begin to fall behind the controls. The thy- 

 roidectomized larvse never do metamorphose, but may develop hind and 

 fore limbs that are larger than those of metamorphosing larvse (Allen (/) ; 

 the fore limbs, however, never break through the skin. This may be due, 

 as suggested by Uhlenhuth (i), to the fact that in the absence of the thy- 

 roid the changes of the skin that lead to the shedding of it do not occur and 

 that consequently the fore limbs are not freed. The ossification of the 

 skeleton is stopped and at the same time growth of the long bones is re- 



