Huxley : Glands and Development 7 



c c 



I 



Metabolism 



T 



B 



JA / 



.' T^ 



T 



Time > 



TISSUE SENSIBILITY AND THYROID FEEDING 



Figure 3. A-A, B-B, C-C, tissue sensitivity (levels of metabolic rate at which break- 

 down of larval organs occur), in Anura, Axolotl, and Necturus, respectively; T-T, 

 maximum metabolic effect possible to produce by thyroid feeding; Ta, Tb. Tc, curves of 

 thyroid growth for Anura, Axolotl, and Necturus, respectively. Only in the case of Anura 

 does the rate of thyroid growth enable it to secrete a sufficient concentration of thyroid to 

 produce metamorphosis. By artificial thyroid treatment it is possible to increase the 

 metabolic rate of Axolotls to the metamorphic point, but the greatest concentration of 

 thyroid that Necturus will tolerate does not serve to break down the larval tissues. 



the result of heavy thyroid doses. It 

 would appear that it is in this way 

 that Necturus, for instance, has be- 

 come permanently larval in form and 

 immune from the metamorphic- effects 

 of thyroid. In Proteus,'" and Nectu- 

 rus/' the histological structure of the 

 thyroid is normal, but its size, espe- 

 cially in Proteus, is relatively small. 

 This in itself will render normal meta- 

 morphosis difficult or impossible, but 

 it will not explain why no amount of 

 thyroid treatment will induce meta- 

 morphosis. This latter fact, it seems 

 to me. is only explicable on the assump- 

 tion that a change has occurred in the 

 other partner, so to speak, in the busi- 

 ness of metamorphosis — the larval tis- 

 sues — rendering them insusceptible of 

 breakdown under increased metabolism. 

 That larval tissues may thus differ 

 in different species in their metabolic 

 relations is shown by the interesting 

 observation of Allen'' that dift'erent 



species of Amphibia when thyroid- 

 ectomized, attain diff'erent degrees of 

 development before differentiation 

 ceases ; thyroidless Bujo Icntigiiwsiis 

 larvae, for instance, attain greater limb- 

 development than thyroidless Raiia 

 pipiens larvae. 



In the Axolotl, the same two pro- 

 cesses are probably at work. There 

 is probably a relative insufficiency of 

 thyroid secretion, and this relative in- 

 sufficiency is due partly to an abso- 

 lutely decreased rate of differentiation 

 and a decreased end-size of the thy- 

 roid itself, partly to a change in the 

 metabolism of the body in general ; 

 but neither of these changes seem to 

 have gone so far as in Necturus. Ex- 

 periments are now being undertaken 

 to test the correctness of this view. 



The longer time necessary to meta- 

 morphose a mature than an immature 

 Axolotl by thyroid feeding, already 

 alluded to, is interesting in this connec- 



