directed to determine this point. This observer found that the proteids 

 of crabs' blood could be injected into the blood stream of the rabbit and 

 undergo assimilation. Being proteids foreign to rabbits' blood, their 

 injection provoked the production in the latter of a precipitin for crabs' 

 blood plasma, but the assimilation of the proteid and the production of 

 the precipitin were found to be absolutely independent phenomena. 



The first group of pharmacological substances may be defined 

 as substances presenting many points of resemblance to proteids, potent 

 like enzymes in infinitesimal doses, and giving rise, as a result of their 

 introduction into the body, to a reaction consisting in the production of 

 an antibody. 



The substances belonging to Ehrlich's second group, which includes all 

 our common drugs, probably act on the protoplasmic molecule or part of 

 it by reason of their chemico-physical properties or their molecular con- 

 figuration. It is difficult to give a more definite expression of their mode 

 of action. We know that in many cases slight changes in the molecule, 

 such as the introduction or withdrawal of an ethyl, methyl, or NH 2 group 

 into or from a drug or group of drugs, alter their physiological actions in 

 a regular manner. We know, moreover, that substances of the most 

 diverse constitution, such as the various anaesthetics, may have little 

 more than their fat solvent powers in common. All these drugs, how- 

 ever, are more or less stable compounds, generally to be obtained in a 

 crystalline form and not easily destroyed by heat. On introduction into 

 the body, the incubation period of their physiological effects is generally 

 determined only by the time necessary for their distribution to, and their 

 diffusion into, the cells which they chiefly affect. Although repeated doses 

 of them can set up a certain degree of tolerance, in no case is there any 

 evidence of the formation of a physiological antidote or antitoxin to the 

 poison. 



To which of these two groups of bodies must we assign the chemical 

 messengers which, speeding from cell to cell along the blood stream, may 

 coordinate the activities and growth of different parts of the body ? 

 The specific character of the greater part of the toxins which are known 

 to us (I need only instance such toxins as those of tetanus and diphtheria) 

 would suggest that the substances produced for effecting the correlation 

 of organs within the body, through the intermediation of the blood stream, 

 might also belong to this class, since here^also specificity of action must 

 be a distinguishing characteristic. These chemical messengers, however, 

 or "hormones" (from oppdw, I excite or arouse), as we may call them, 

 have to be carried from the organ where they are produced to the organ 

 which they affect, by means of the blood stream, and the continually 

 recurring physiological needs of the organism must determine their 

 repeated production and circulation through the body. If they belong 

 to the first class and are analogous to the toxins, the production of a 

 given substance and its discharge into the blood stream must give rise to 

 the formation of a specific antibody, which must increase in amount with 

 each production of the substance in question and tend therefore to 



