METAMORPHOSIS OF NEOTENOUS AMPHIBIANS 415 



One eng;rafted animal died February 25th. The experiment 

 was abandoned jMarch 10th, as some of the transplanted individ- 

 uals had fore hmbs. The data clearly indicate that the thyroid 

 glands of neotenous tadpoles with hind legs 8 to 10 mm. are 

 physiologically active and capable of inducing acceleration of 

 limb growth and development when transplanted into other 

 neotenous animals of similar size and developmental stage. 

 However, metamorphic change following grafting is not so rapid 

 or marked as when the same type of gland is implanted in im- 

 mature (non-neotenous) larvae. But in either case the absorp- 

 tion of the secretion of the grafted gland induced a greater reac- 

 tion than the same gland is capable of producing (in the same 

 time and under similar conditions) when left unmolested in the 

 neotenous animal of which it was originally a part. 



SUMMARY OF CONCLUSIONS 



1. Adult Necturus were fed, injected, and engrafted with 

 physiologically active thyroid substance with negative results. 

 The injection of 20 mg. of thyroxin iodine had no effect upon 

 metamorphosis, whereas a 1 to 50,000 solution of this compound 

 readily metamorphosed immature larvae of Amblystoma. The 

 experiment indicates that perennibranchiate amphibians are 

 permanent larvae and have lost the ability to transform under 

 the stimulus of thyroid treatment. 



2. Necturus differs markedly from the axolotl in its reaction to 

 thyroid administration, as the latter readily metamorphoses when 

 fed or injected with this substance. 



3. The thyroid glands of the perennibranchiate Necturus are 

 quite variable in size, and in some animals may occur as large 

 vesicles more or less isolated from one another. In other in- 

 dividuals the glands are rather small and may consist of but four 

 to six extremely large vesicles. In Necturus the thyroids are 

 generally located near the apex of the triangle formed by the 

 geniohyoid and external ceratohyoicl muscles. 



4. Despite its larval characters, Necturus possesses thjToid 

 glands of great physiological activity, as shown by heteroplastic 

 transplantation into immature anuran larvae. 



