METAMORPHOSIS OF NEOTENOUS AMPHIBIANS 403 



gland, but I do not find thyroids." This is the only mention 

 made of the thyroids in her paper. Recently, through courtesy 

 of Prof. H. H. Wilder, the writer had an opportunity of examining 

 Miss Emerson's material, consisting of serial sections through the 

 head of one animal. No trace of a thyroid was observed, but 

 it should be stated that some of the epithelial structures had 

 disappeared. 



The writer has carefully examined three adult specimens of 

 Typhlomolge and failed to find any trace of a thyroid. The entire 

 lower-jaw region, back to and including the heart, was dissected 

 under a high-power binocular microscope, and some tissue 

 sectioned, but with negative results. However, the failure to 

 find the glands does not necessarily mean that they were not 

 present or had not been present at some period, because the animals 

 had been preserved over fifteen years in alcohol and many of the 

 epithehal structures had undergone disintegration during this 

 interval. 



If the thyroid mechanism of Typhlomolge is congenitally 

 lacking, then this amphibian is the only vertebrate known in 

 which the gland is normally absent. It has been stated in the 

 literature that Typhlomolge is only the neotenic larva of Spelerpes, 

 but it is well known that Spelerpes larvae possess thyroids. 

 It seems probable that the Texas cave salamander has a thyroid, 

 but that it develops as a diffuse aggregation of follicles, somewhat 

 similar to the condition known to exist in teleosts. At any rate 

 the question of the presence or absence of the gland deserves 

 further investigation before it can be accepted as an established 

 fact, because the history of vertebrate morphology is replete with 

 descriptions of forms supposedly anomalous for the lack of certain 

 structures, only to be later shown to possess them. 



To sum up, it may be said that these experiments indicate that 

 Necturus and probably other perennibranchiates have per- 

 manently lost their ability to metamorphose into terrestrial forms 

 under the stimulus of thyroid administration alone: our experi- 

 ments indicate that the thyroid apparatus of these animals is 

 highly active and potent despite their larval characters. 



