Significance of the Internal Secretory System 147 



tion, as Allen's results directly prove. 



A defect in Loeb's reasoning is his ignoring the truth that 

 thyroid substance lacks in the case cited the prime attribute 

 of a sufficient cause, namely full competency. The actual 

 substance which enters into the new legs, and as far as that 

 goes, into the other new parts, is probably provided very 

 little, if at all, by the specific substance or hormone of the 

 thyroid. The causal role played is the relatively humble one 

 of excitor or stimulator to an activity not essentially new 

 but only exceptional as to time. Of course Loeb does not 

 need to be told that hormones incite growth rather than 

 provide the substance itself out of which the organs and 

 parts are built. Indeed portions of his account of this very 

 case show positively that he is aware of this. We read: 

 "Thus we see that the mesenchyme cells giving rise to legs 

 may lie dormant for months or a year but will grow out 

 when a certain type of substances, e.g., thyroid, circulates in 

 the blood. There may exist an analogy between the activ- 

 ating effect of the thyroid substance and the activating 

 effect of the spermatozoon or butyric acid (or other par- 

 thenogenetic agencies) upon the egg." 



This suggestion of analogy between the action of thyroid 

 secretion and "butyric acid (or other parthenogenetic agen- 

 cies)" is well taken. The resemblance between the two 

 agencies, as judged by their effects on development, is cer- 

 tainly rather close. Very well; would Loeb, then, call bu- 

 tyric acid organ-forming substance, and identify it with the 

 "formative stuff" of Sachs ? Certainly any substance which 

 will rouse the latent developmental capacities of an egg into 

 activity is in a minor sense formative, especially if these 

 capacities are wholly unable to start without some such 

 agency. But since butyric acid, or some one of the other 

 dozens or scores of parthenogenetic agencies, may activate 

 the eggs of many, many species of animals ; and since the 

 eggs of many, many species may be activated by any one of 



