Experiments with Thyroidectomy in Amphibia. 103 



the larvae averaged 18 mm. in length (225 experimental and 182 

 control animals) the experimental larvae grew more rapidly than 

 the controls, and most of them became ultimately much larger 

 than the controls, the largest being 72 mm. in length as against 

 50 mm. for the controls. Thirty thyroidless animals failed to 

 metamorphose and in twelve others the thyroid regenerated, per- 

 mitting metamorphosis at the normal time. The rest of the 

 experimental larvae were killed or died before this time. Six of 

 the thyroidless larvae are still alive, nine months after the normal 

 time of metamorphosis. They practically ceased growing six 

 months ago. 



In the absence of the thyroid the secretory (anterior) portion 

 of the hypophysis underwent physiological hypertrophy to com- 

 pensate for the loss of the thyroid, as both normally are concerned 

 with metabolism, although not in an exactly similar way. This 

 hypertrophy probably accounts for the gigantism and infantilism 

 (retention of the larval form) of the thyroidless larvae. Normally, 

 synapsis begins in the testis after metamorphosis, and in the ovary 

 a short time before metamorphosis. In our experimental animals 

 so far examined, synapsis in the testis was practically completed 

 at the time of metamorphosis of the controls and differentiation of 

 the ovary also was hastened greatly. The hypertrophy of the 

 hypophysis probably accounts for this sexual precocity. The 

 growth in size of the gonads continues with that of the other parts 

 of the body and is independent of the changes normally occurring in 

 metamorphosis. Both processes, however, depend upon the general 

 metabolism. Failure of metamorphosis is due to abnormal meta- 

 bolism, brought about, either by loss of normal thyroid secreton 

 or by abnormal secretion of the hypophysis which is affected by re- 

 moval of the thyroid. Both causes may be involved, and also, the 

 secretion of still other glands may be changed after loss of thyroid 

 function. Failure of metamorphosis may depend directly upon 

 an abnormal calcium metabolism with which the thyroid is believed 

 to be concerned, since the initiation of metamorphosis is probably 

 brought about by growth of the skeleton. The condition in 

 thyroidless larvae may be neotony, as Allen has called it. The 

 hypophysis of axolotls should be investigated to see if it is the 

 cause of failure of metamorphosis in these animals. In the obser- 



