SKIN TKANSPLANTATION IN FROG TADPOLES 393 



Recently L. Loeb ('20, '21) has summarized the investigations 

 that have been carried on by him and his co-workers for the past 

 nineteen years. Having performed several kinds of transplanta- 

 tions on guinea-pigs, rats, rabbits, and fowl, he concludes that 

 all the tissues of an animal contain a specific chemical group, 

 which is responsible for the animal's individuality. He calls 

 this substance the 'individuality differential.' When trans- 

 planted to a closely related animal of the same species or to one 

 not closely related or to an animal of another species, this specific 

 substance reacts with the individuality differential of the host, 

 either directly or indirectly, to form a syngenesio-, a homoio-, 

 or a heterotoxin, respectively. The resulting reaction may be 

 studied quantitatively, and is found to increase in violence as the 

 remoteness of the relationship between graft and host increases. 

 This furnishes, therefore, a method of determining the degree of 

 relationship between individuals. In addition to this individ- 

 uality differential, there are also in the tissues of an animal 

 ''contact substances," "which act upon adjoining or distant parts 

 of the body and thus bring about a correlation of functions which 

 makes possible the orderly development and maintenance of the 

 organism" (Loeb '21, p. 163). He concludes that cells and tis- 

 sues are constantly producing certain substances which play dif- 

 ferent roles in the life of the organism as a whole. Some of them, 

 for example, are concerned with preserving the individuality of 

 the whole animal; others act as stimuli upon adjoining tissues 

 and organs; and still others are carried to distant parts of the 

 body before exerting their specific action. The latter are the 

 hormones. 



The evidence in favor of specificity of protoplasm due to chemi- 

 cal differences not only between different individuals, but also 

 between different organs and regions of the individual is fairly 

 conclusive. The subject is a part of modern biological thought, 

 and only a few of the researches concerned with it have been 

 mentioned here. However, the possibility that the common 

 tissues, such as muscle, connective, and skin, might contain 

 specific substances in different regions of an individual has re- 

 ceived slight attention. Many experiments have shown marked 



