ACTIVATORS, KINASES AND HORMONES 189 



travel in the blood stream to other organs of the body and 

 effect correlation between the activities of the organ of 

 origin and the organs on which they exert their specific 

 effect. Such substances belong to the crystalloid rather 

 than the colloid class; they therefore are thermostable and 

 do not act as antigens when injected into the living ani- 

 mal. The general idea of this definition is clear and most 

 suggestive, but in its details it is made especially to suit 

 the case of secretion, and therefore may not fit so well for 

 other substances of like physiological value. Conveyance 

 through the blood stream, while certainly the most common 

 occurrence for this class of bodies, ought not to constitute 

 an essential part of their definition. The secretin formed 

 in the intestinal epithelial cell is conveyed to the pancreas 

 in the blood and brings about a correlation between the 

 activity of this gland and that of the duodenum, but on 

 the other hand some substance contained in the pancreatic 

 juice and conveyed to the duodenum in the stream of secre- 

 tion excites the formation of the enterokinase, and thus 

 correlates the activity of the duodenum with that of the 

 pancreas. The two actions seem to be so similar, except 

 for the means of transport, that one would naturally put 

 them in the same class. By the same reasoning we might 

 be justified in designating the hydrochloric acid of the 

 gastric juice as a hormone in reference to its action in caus- 

 ing a formation of secretin in the epithelial cells of the 

 duodenum. One can imagine that a similar transporta- 

 tion may occur in the secretions of the reproductive or 

 respiratory passages, in the cerebro-spinal fluid, as seems 

 to be the case for a time at least with the secretion of the 

 pars intermedia of the pituitary gland, or even along the 

 axial stream of a nerve fiber. If, as seems to me, the idea 

 of correlation or coordination is the essential point rather 

 than the assumption that the product must constitute an 

 internal secretion, we might modify the definition so far 

 as to designate as hormones those substances in solution 

 which, conveyed from one organ to another through any of 



