44 



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



Great diversity in the rate of metamorphic change was found, so much so 

 that the original purpose of the experiment — viz., to get some quantitative 

 data on the time-relations of metamorphosis to iodine concentration — 

 had to be given up. It appeared that the prominence of the eyes 

 which accompanies metamorphosis in these forms, might begin at high 

 temperatures while the gills were still hyper-normal in size. Thus the 

 effect of high temperature may be to mask the effect of iodine as far as the 

 gills are concerned. When the resorption of the gills did begin at high 

 temperatures, it went very quickly, the effect of the iodine overcoming the 

 antagonistic effect of temperature. This is probably, according to Swingle, 

 due to the power of all tissues of the body, but especially the thyroid, to 

 manufacture from iodine some substance which causes the initiation of meta- 

 morphosis, when it reaches a certain critical concentration inside the body. 



It, however, appears probable that the iodine may also have a direct effect 

 on the tissues of the gills. The violent action of the l/10th solution upon 

 the filaments in the first experiment is very likely to be explained in this way. 

 It is at any rate certain that the filaments are very sensitive to external 

 agencies, as is also the tail-fin. Both of these structures, for instance, are 

 much reduced when Amblystoma larvae are kept in a very small amount of 

 water or in damp moss, the mechanical alterations seeming to initiate the 

 reduction (although of course the final complete metamorphosis which takes 

 place in these circumstances must depend upon other, more deep-seated 

 changes). We may say, therefore, that low temperature, exposure to air 

 instead of water, and probably iodine solutions, have a directly unfavourable 

 effect upon the gill-filaments, causing a certain amount of dedifferentiation 

 and resorption, while the other agencies, such as thyroid diet and iodine 

 accumulated in the body, exert an indirect effect by altering the internal 

 environment to the point where metamorphic changes are started. Once these 

 begin, the character of the gill-epithelium is altered, and the gills are rapidly 

 reduced to mere blobs (fig. 4). 



3. Negative Effects uf Administration of other Ductless Glands and of Iodine on 



Metamorphosis in Axolotls. 

 Attempts were made to induce metamorphosis by administration of iodine 

 in the medium and with the food to three animals 12 cm. in length. In the 

 latter case, as in Swingle's earlier experiments (1919), a small quantity of 

 finely powdered iodine was used, being in this case dusted on to thin slices 

 of meat rolled into pellets. Such treatment does not produce obvious dis- 

 comfort, nor is it poisonous, and with several animals was continued bi-weekly 

 for 8 to 10 weeks without any diminution of gills and tail-fin or any evident 



