METAMORPHOSIS OF NEOTENOUS AMPHIBIANS 399 



lacking in the thyroid apparatus, which, if true, accounts for the 

 suppressed metamorphosis. It will be recalled that it is possible 

 to induce neoteny in anuran larvae by thyroid extirpation. 



This evidence is suggestive, and taken in conjunction with 

 what we know of the thyroid gland and its relation to metamor- 

 phosis suggests that the thyroid mechanism of forms such as 

 Necturus, axolotl, and neotenous anuran larvae is defective and 

 incapable of bringing about transformation. 



EXPERIMENTS ON PERENNIBRANCHIATES (NECTURUS 

 MACULATUS) 



A group of adult necturus were obtained from the Ohio Valley 

 and repeatedly injected with thyroid extract, and at the same 

 time forcibly fed large quantities of the desiccated commercial 

 preparation by means of a long glass pipette thrust down the 

 throat. This procedure was repeated several times with negative 

 results, despite the fact that the thyroid dosage was relatively 

 enormous. The physiological activity of the thyroid prepara- 

 tion was tested by feeding small amounts to larvae of R. clamata, 

 averaging 50 mm. total length, but without hind legs. These 

 animals promptly showed marked indications of metamorphosis 

 within eight days from the date of feeding (fig. 7) . The iodine 

 content of the desiccated tissue was given on the label of the 

 bottle as 0.21 per cent by weight and the analysis made by the 

 chemists of Parke, Davis & Company, Detroit. It is obvious 

 that the thyroid preparation used in the experiment was not 

 responsible for the negative results. One animal was injected 

 twice intraperitoneally with 10 mg. of thyroxin iodine obtained 

 from the laboratories of E. R. Squibb & Sons, and then forcibly 

 fed large quantities of desiccated thyroid and anterior pituitary 

 lobe substance. The physiological activity of the thyroxin iodine 

 was tested by placing two young specimens of Amblystoma 

 punctatum in a 1 to 50,000 solution, whereupon they metamor- 

 phosed within two weeks. None of these agents, singly or taken 

 together, produced the slightest indications of metamorphosis, 

 nor, I may add, appeared to harm the Necturus in any way. 

 As a last resort, three thyroid glands of newly metamorphosed 



