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SCIENCE PROGRESS 



cells, of a number of tissues. He found that in some organs, 

 such as the limbs, the coefficient was accelerated — i.e. in- 

 creased at a regular rate until metamorphosis. In others, 

 such as the gills or fin, it was retarded, and degeneration 

 finally set in ; while in still others, such as certain portions 

 of the skin, no change was observed. Even in one and the 

 same tissue, the most diverse conditions might obtain ; thus 

 in the ectoderm of the head and stomodeum, some regions were 

 unaffected, others were found to respond by increased growth, 

 others by degeneration, although all three were previously 

 indistinguishable histologically. In other words, different 

 tissues are differently sensitised to the thyroid hormone, the 

 optimum concentration of thyroid being low for some and high 



Fig. I. 

 A A. Growth of larva, reduced to common scale. 



T1-T5. Curves of thyroid growth (thyroxin concentration) in examples A1-5. 

 M1-M4. Points at which metamorphosis occurs. 



for others, while still others are not affected by considerable 

 changes. 



Thus the most varied combinations of tissues and thyroid 

 are possible. In passing, we may note that the fact that the 

 limbs of Anura are accelerated in their development by thy- 

 roid while those of Urodela are not, implies that those of 

 Anura have become sensitised to thyroid concentration late in 

 phylogeny ; the tail of Anura has also become sensitised at 

 a late evolutionary period, but to degeneration instead of 

 to increased activity. For the present, however, we may 

 neglect these variations in separate organs, and focus our 

 attention on the metamorphic change as a whole. This, as 

 I have elsewhere indicated, implies (i) the existence of certain 

 tissues, such as the fin and gills, which have a low optimum 

 metabolic rate, and degenerate if the thyroid concentration 

 is raised above a certain point, (2) the existence of other tissues, 



