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 
