Significance of the Internal Secretory System 145 



of the tadpoles resulted. "Towards the end of metamor- 

 phosis the animals hardly moved about in the water. They 

 were always lying quietly, generally on their backs. When 

 disturbed they would move for a few seconds in a somewhat 

 convulsive manner and then drop again to the bottom of 

 the dish, while tadpoles fed on other material would swim 

 about for a long time." 5 So mammalian thyroid substance 

 is not only organ-forming for a whole series of frog organs 

 but it is habit-forming for a variety of frog habits ! 



But we must not let the ludicrousness of this veer us 

 away from the reasoning in the case. Taking the facts 

 actually brought out by Gudernatsch, what becomes of the 

 specificity of the substance, which according to Loeb's state- 

 ments w r as what Sachs hypothesized? "Sachs suggested that 

 there must be in eacli organism as many specific organ-form- 

 ing substances as there are organs in the body." The truth 

 appears to be that thyroid substance in this case is organ- 

 forming in much the same sense that water is organ-forming 

 for the leaves, flowers, and fruit in a squash vine, which could 

 not develop without water. Indeed the analogy suggested 

 goes further than appears at first sight. As everybody 

 knows, the effect on young plants of a scant water supply 

 is to stunt the plant as to size and to hasten its blossoming 

 and fruiting. That is, an under-supply of w r ater has an 

 effect on immature plants similar to that of an over-supply 

 of thyroid substance on immature frogs, namely that of re- 

 tarding growth and hastening metamorphosis. The total 

 effect in each case is systemic. In other words, the real 

 significance of the instance used by Loeb is just the opposite 

 of his interpretation of it. Thyroid substance is organ- 

 forming only through being organism- trans forming. Full 

 justification of this way of interpreting the part in 

 development played by thyroid substance is furnished by the 

 recent studies of B. M. Allen. This investigator lias, like 

 Gudernatsch, experimented with frog larvae. He has, how- 



