48 



Messrs. J. S. Huxley and L. T. Hogben. 



(ii) That their thyroid mechanism is unable to make use of the available 

 iodine supply so as to produce the requisite amount of active iodine compound 

 required to stimulate metamorphosis. 



(iii) That endogenous factors involved in the maintenance of the thyroid 

 in a condition of functional activity are not operating effectively. 



(iv) That the larval tissues concerned have collectively lost the power to 

 respond to the thyroid activator. 



The second and last possibilities are emphasised by the fact that, whereas 

 in normal Amphibia Swingle has shown that iodine free of organic combina- 

 tion can suffice to induce metamorphosis not only in normal but in thyroid- 

 ectomised larvse, in the case of the mature Axolotl, iodine alone is not an 

 efficient substitute for the thyroid autacoid. The importance of the third is 

 sufficiently demonstrated by the work of Uhlenhuth (1919) on the relation 

 of growth to metamorphosis in Salamanders, and Allen's recently published 

 account of tbe inhibition of transformation in Anura by hypophysectomy. 

 The state of affairs encountered in the Mexican Salamander is eminently 

 suitable to a further analysis of what we have termed the endogenous factors 

 in Amphibian metamorphosis ; and it is hoped to obtain shortly in this 

 connection data respecting the relation of the method of thyroid-feeding to 

 enforced air-breathing as a means of bringing about the assumption of adult 

 characters. 



5. Pigment Reactions of Axolotls to Pituitary and Adrenal Hormones. 



While the feeding of Axolotls with pituitary gland did not prove productive 

 of positive data in relation to metamorphosis, it yielded results which 

 encouraged further enquiry into the reaction of Amphibian melanophores to 

 the internal secretions. 



A few hours (6-12) after feeding Axolotls on pituitary (whole) gland, a 

 marked darkening of the skin was observed ; this was not noticed the first 

 few days of pituitary diet, but became increasingly pronounced as the feeding- 

 continued. At first the darkening was very gradual, attaining its maximum 

 intensity about 18 hours after feeding, which took place every 48 hours. 

 Curiously enough, when this effect had quite passed off by the morning of 

 the second day, the animals were of a ghostly pallor — considerably lighter 

 than their normal shade, and remained thus until fed again. The rate of 

 darkening progressively increased ; after 3-4 weeks of the pituitary diet the. 

 maximum degree of expansion of the pigment cells was reached within an 

 hour. The extreme subsequent paleness still occurred, appearing within 

 24 hours of feeding. Finally, after about 3 months' treatment the response 

 diminished. 



