THE PARS BtrCCALlS OF THE HYPOPHYSIS 89 



The conditions then obtaining in these three classes of tad- 

 poles seem to indicate that the epithelial fragment where sep- 

 arated from the true neural lobe, either in itself or through its 

 reaction with brain tissue, other than the true neural lobe, is 

 able to stimulate the thyroid to normal development, but that 

 under these conditions metamorphosis cannot be completed; 

 that when this'fragment is in very slight contact with the neural 

 lobe an hjrpertrophied thyroid results and complete metamor- 

 phosis is effected, and that when the usual relations of a com- 

 pleter contact between the two parts of the hypophysis obtain 

 no hjrpertrophy of the thyroid occurs and we have the comple- 

 tion of the metamorphic processes. It might thus appear that 

 the activity of the thyroid is thus the sole or determining factor 

 on which the metamorphic processes are dependent. That such 

 may not be the case seems clear from evidence gained by thyroid 

 feeding. In thyroid and thyroxin feeding extending for periods 

 in excess of six weeks and during which time daily minute doses 

 of thyroid or thyroxin were administered, I have not been able 

 to bring to complete metamorphosis a typical hypophysis-free 

 albinous tadpole. The early stages in this process were passed 

 through (leg growth, partial tail absorption), but the tadpole 

 could not be carried past a certain stage, death invariably super- 

 vening. This appears to be the case also with the 'partial' 

 albinos of class 1, whose metamorphic processes could not be 

 carried to completion nor materially advanced after the onset 

 of the metamorphic stasis by instituting a carefully controlled 

 thyroid dosage. The results secured from the feeding of thyroid 

 to completely and partially hypophysectomized tadpoles stand 

 in sharp contrast to similar feeding experiments with the thy- 

 roidectomized tadpoles, which were readily completely meta- 

 morphosed by this treatment. It is not to be denied that a 

 more carefully regulated or a different dosage of thyroid might 

 bring about complete metamorphosis in the albino, but that it 

 would do so appears improbable from the evidence at hand. 

 The evidence thus indicates that metamorphosis is dependent 

 upon the active principle of at least two glands; in the absence 

 of either, the larval condition persists. One gland, the thyroid, 



