156 The Unity of the Organism 



his discussion explicitly identifies internal secretions. "At 

 that time the idea of the existence of such specific organ- 

 forming substances was received with some skepticism. . . . 

 Such substances are known now under the name 'internal 

 secretions' or 'hormones' ; their connection with the theory 

 of Sachs was forgotten with new nomenclature." Then 

 follows the reference to the tadpole legs ; so that were consis- 

 tency really a jewel to the author, he could not escape mean- 

 ing that the substance of the legs was actually derived di- 

 rectly from the particular internal secretion in question, in 

 this case thyroid substance ; and the later statement about 

 the formation of legs from dormant mesenchyme cells 

 through the mere activating effect of thyroid substance is by 

 implication contradicted. 



This brings up again a matter about the interpretation 

 of development which we have dwelt upon in several other 

 connections, that of protest against rejecting the indubi- 

 table evidence of the senses in favor of a priori conceptions. 

 The crucial question in the present case is this : which is the 

 more fundamentally organ-forming substance for frogs' legs, 

 the mesenchyme cells which "though giving rise to legs may 

 lie dormant for months," or the thyroid substance which 

 may stimulate these cells into premature activity? While 

 Loeb's discussion does not raise this question definitely, the 

 implication is unescapable that thyroid substance is for him 

 the more fundamental. What else is the meaning of the 

 contention that this substance is organ-forming while no- 

 where do we find the mesenchyme cells so designated? Yet 

 the observational evidence is that the production of legs is 

 accomplished through the transformation of mesenchyme 

 and other cells which in the larva are not leg-substance, but 

 in the adult are leg-substance. Hence it follows that so far 

 as actual observation is concerned the mesenchymatous and 

 other larval substances are more entitled to be called organ- 

 forming than is the thyroid substance. 



