8 



The Journal of Heredity 



THREE WEEKS OF THYROID 

 FEEDING 



Figure 4. Control and thyroid-fed tad- 

 poles after twenty-three days of thyroid 

 feeding show the terrific acceleration in de- 

 veloipment that an increased concentration of 

 thyroid secretion produces. Accompanying 

 these external changes are even more re- 

 markable internal ones. In two weeks, the 

 undifferentiated alimentary of the tract of 

 the tadpole decreased in length 246 mili- 

 meters (nearly ten inches), and from a sim- 

 ple tube differentiated into stomach, in- 

 testine, and the other parts constituting the 

 digestive system of the normal adult frog. 

 Photograph reproduced by permission of Dr. 

 \V. W. Swingle and the Journal of Experi- 

 ment al Znoloi/y. 



tion. The mature ti.s.sues, it would 

 appear, do not break down so readily 

 as the immature under the action of 

 thyroid secretion. A rough diagram, 

 which is intended only as a graphic 

 symbol, may help clarify this argu- 

 ment. ( Fig. 3.) 



If we accept this view, we might 

 expect to find that Anura, as the group 

 in which metamorphosis is the most 



specialized, show an increased sus- 

 ceptibility of their larval organs to 

 thyroid concentration. That this may 

 possibly be so is shown by Uhlen- 

 huth's cHscovery"* that iodine in some 

 Urodeles at least, is not efficacious in 

 bringing" about metamorphosis. 



The fact that Axolotls and other 

 Urodeles do not metamorphose under 

 simple iodine treatment would then 

 indicate, as suggested above, that their 

 larval tissues are less susceptible to 

 increase in basal metabolism, so that 

 the limited amount of increase which 

 iodine can Ijring about in the thyroid, 

 even when free iodine is present in 

 the greatest amount which the animals 

 will tolerate, is insufficient to do what 

 larger quantities of thyroid can effect. 



Use of the Thyroid a Matter of 

 Convenience 



It is further clear that normal utiH- 

 zation of the thyroid by Amphibia as 

 an initiator of metamorphosis is a 

 matter of convenience, so to speak, not 

 of necessity. When metamorphosis 

 does occur, and there is therefore a 

 marked consecutive dimorphism, the 

 general problem is priinarily of caus- 

 ing the larval tissues to break down 

 at a given time, and secondly, of caus- 

 ing the more rapid growth of adult 

 tissues. Whatever change of external 

 or internal environment iriay be found 

 which will promote this breakdown, 

 mav be utilized to promote metamor- 

 phosis. 



In insects it is known that the whole 

 type of metabolism alters at meta- 

 morphosis, indicating a pronounced 

 qualitative change of internal environ- 

 ment. In Echinoids, the writer has 

 recently suggested that the sinking of 

 the larvae to the bottom, as the result 

 of the increasing weight of the Echinus 

 rudiment, may be the initiatory factor 

 in metamorphosis, since the benthic 

 environment is im favorable to the lar- 

 val tissues, adapted as they are to the 

 totally different conditions of plank- 

 tonic life.'" If this should be sub- 



