GROWTH OF AMPHIBIA AFTER THYROIDECTOMY 47 



Thyroidless larvae do not metamorphose even when kept alive 

 for a year after the metamorphosis of the controls, and hence 

 are probably unable ever to metamorphose at all. 



Failure in metamorphosis is attributed to faulty metabolism, 

 especially of calcium, since one of the most striking effects of 

 thyroid removal is a deficiency in calcification and ossification 

 of the skeleton. 



Older thyroidless larvae will live for a time out of water, if 

 kept in a moist chamber. They decrease rapidly in volume and 

 their tails shorten considerably, but the skeleton does not grow. 



Thyroidless larvae are able to breathe with their lungs, which 

 develop normally. 



Thyroidless larvae retain the power of regeneration of lost 

 parts to a limited extent for more than a year at least. 



The brain grows more slowly in thyroidless than in normal 

 larvae, both actually and relatively. It differentiates slowly 

 and becomes ultimately much larger than the fully differentiated 

 normal brain, while still only partially differentiated. In thy- 

 roidless larvae kept alive a year the brain compared with young 

 frogs becomes fully differentiated in shape, excepting the anterior 

 end of the hemispheres. The brain of the normal animal in- 

 creases in relative size during metamorphosis. 



The liver of normal animals changes in shape during meta- 

 morphosis and decreases in size. In thyroidless larvae the liver 

 becomes relatively very large and tends to assume the mature 

 shape, but never quite differentiates because a coil of gut is 

 half embedded in it. 



The hypophysis undergoes hyperplasia after removal of the 

 thyroid, but is unable to compensate entirely for the loss of the 

 latter. This hyperplasia may account for the rapid growth of 

 thyroidless larvae. The hypophysis normally increases in rela- 

 tive size during metamorphosis. 



The thymus persists in older thyroidless larvae. In thyroid- 

 less and control larvae of the same size the thymus glands are 

 of the same size. The thymus is relatively larger in young 

 frogs than in larvae, since it decreases less during metamorphosis 

 than does the body as a whole. In larger thjToidless larvae the 



THE JOURNAL OP EXPERIMENTAL ZOOLOGY, VOL. 29, NO. 1 



