AMPHIBIAN METAMORPHOSIS 



6ii 



such as the limbs, which have a high optimum, and increase 

 their rate of development as the thyroid concentration in- 

 creases. Then we have the following chief possibilities to 

 consider : (A) (fig. i) * Relative rate of thyroid develop- 

 ment variable, (i) Very rapid development of the thyroid; 

 early metamorphosis at a small size : e.g. Bitfo lentiginosus. 

 (2) Moderately rapid development of thyroid ; metamor- 

 phosis in late summer : Rana teniporaria, R. tigrina. {3) Slow 

 thyroid development ; metamorphosis only in the second 

 season : R. clamitans. (4) Very slow thyroid development, 

 metamorphosis in the third season, at a very large size. Rana 

 catesbeiana (American bull-frog). (5) Thyroid's maximum 

 size below that needed for a thyroxin concentration capable 

 of including metamorphosis. Probably Typhlomolge, one of 

 the Perennibranchiates. (6) Thyroid absent, as in thyroid- 



Tm 



L, 



Fig. 2. 



T1-T3, curves of thyroid gro%vth, and Li-Ls, levels of larval tissue, suscepti- 

 bility to thjrroid, for normal Urodele, Axolotl, and Perennibranchiate respectively. 

 T,n, maximum concentration of thyroid to be produced in the body. 



ectomised tadpoles. Total absence of metamorphosis ; growth 

 to a very large size. 



(B) (fig. 2) Thyroid developmental rate and maximum re- 

 maining approximately constant, (i) Tissues highly sensi- 

 tive ; probably Anura. (2) Tissues sensitive : probably most 

 Urodela. (3) Sensitiveness of tissues below threshold values 

 needed to bring about metamorphosis. Probably Necturus. 



(C) Both thyroid and tissue sensitiveness varying ; e.g. in 

 the Axolotl probably the tissue sensitiveness is reduced, as is 

 also the rate of thyroid development and the maximum con- 

 centration attained by its hormone. 



It is obvious that such a scheme is provisional and tentative 

 only, especially as regards the particular examples given ; but 



^ The figures are intended only as graphic symbols, not as representing 

 actual quantitative relations. 



