186 E. UHLENHUTH 



tarded (Terry). The thymus gland, though its structure develops like in 

 a normal larva, does not migrate to the position that it has in adult frogs, 

 nor does it degenerate like that of a normal frog. The sex glands show a 

 behavior opposite to that observed in thyroid-fed larvae ; while most of the 

 organs (coiled intestinal tube, brain, mouth, skeleton, thymus, etc.) ex- 

 hibit a marked retardation or even complete inhibition of development, the 

 sex glands develop exactly like those of normal larvae (Allen (#)). This 

 behavior corresponds well to that of several species of salamanders that 

 normally remain permanently larval but develop the sex organs to a mature 

 stage and breed while larvae. This shows again that the development of the 

 sex organs is not controlled by the thyroid hormone, at least in the amphi- 

 bians. Similarly, the skin, in many species of salamanders, may develop 

 even though development of the rest of the body is checked (Uhlenhuth 

 (i)). Particularly interesting in this respect is the behavior of the eyes. 

 In man protrusion of the eyes is abnormal and results from a disturbed 

 function of the thyroid (exophthalmus in Basedow's disease). In amphi- 

 bians, on the other hand, exophthalmus is a normal characteristic, which 

 begins to appear as a rule at the time of metamorphosis. But it may also 

 develop in animals that for some reason do not metamorphose, as, for in- 

 stance, in thymus-fed salamander larvae. Kaufman (&) has therefore sug- 

 gested that the exophthalmus may not be produced by the normal thyroid 

 secretion, but by the escape of some toxic substances from the thyroid in 

 the absence of the normal process of the elaboration of the hormone (see 

 also Uhlenhuth (/*,)) 



While in mammalians athyroidism is pathologic, in one American 

 species of amphibians, Typhlomolge railibuni, no thyroid is found (Em- 

 merson). This animal, which inhabits certain cretaceous caves in Texas, 

 behaves as we would expect from the results of the extirpation experi- 

 ments ; it remains, during its entire life, in a larval condition and many 

 of its organs are only poorly developed or atrophied; yet the sex organs 

 develop normally and reach maturity while the rest of the animal is in 

 an infantile condition. The Typhlomolge is of special interest, also, 

 because it demonstrates at what a phylogenetic stage the amphibians and 

 perhaps the entire higher vertebrates would be if they had not developed 

 the thyroid mechanism. For this reason I cannot agree with Smith, who 

 thinks that the thyroid hormone in causing metamorphosis does not pro- 

 duce anything new, but simply by increasing the metabolism merely 

 accelerates a process that would occur in any case, but at a slower rate. 

 On the contrary, even if the thyroid hormone does not do anything else 

 but change the metabolism in a quantitative way, the Typhlomolge as well 

 as the thyroidectomizod tadpoles show that this mere quantitative change 

 is able to create in a definite physicochemical system supplied by heredity 

 new morphological characters that could never have been developed in the 

 absence of the thyroid mechanism. 



