CAN ORGANISMAL DIFFERENTIALS BE CHANGED? 581 



stronger if the transplant is allowed to remain in the host for several 

 months, as a rule actually becomes weaker, and also the reaction of the 

 connective tissue decreases considerably in intensity in the course of time. 

 We may assume that this decrease is due either to changes taking place in 

 the host, which becomes accommodated to the strange individuality differ- 

 ential of the transplant and therefore reacts less strongly to it, or to similar 

 changes in the transplant, which ceases to produce the differential with full 

 strength. These changes would therefore be of an adaptive character, or 

 they might be due to an injury to the transplant, resulting from the long- 

 continued action of a strange environment ; however, we would not, under 

 these conditions, have to deal with actual alterations of the organismal 

 differentials, but merely with certain modifications of their manifestations. 



2. A much further-reaching change in the nature of the organismal 

 differentials has been assumed by Rhoda Erdmann and Gassul to occur if 

 amphibian skin, in a first period, is cultivated for some time in vitro and then 

 transplanted ; they believe that under these conditions it is possible to alter 

 the individuality as well as the species differentials of the transplant and to 

 make it more similar to that of the host and thus to improve the chances of 

 successful transplantation. For this purpose these investigators cultivated 

 skin for a considerable length of time, first in plasma and tissue extract of 

 its own species, then, step by step, they changed the type of plasma and 

 extract, until it approached more nearly in constitution that of the host 

 organism ; they believed that by this procedure they had succeeded in increas- 

 ing the compatibility between transplant and host, and moreover, it was 

 found that the greater the distance in relationship between host and trans- 

 plant, the longer the time required to effect the transformation of organismal 

 differentials through preliminary growth in vitro ; this interval could there- 

 fore serve as a measure of the nearness or distance of relationship between 

 host and transplant. 



In the first experiment of this kind Rhoda Erdmann cultivated embryonal 

 skin of birds in vitro for from ten to twelve days in homoiogenous plasma, 

 and after transplantation of this tissue into defects in the skin of living 

 homoiogenous adult hosts it was observed that the transplant remained alive 

 longer than corresponding tissue that had not been explanted previously. 

 Moreover, the graft no longer called forth as strong a lymphocytic and 

 fibroblastic reaction as does ordinary homoiotransplanted avian embryonal 

 skin. Rhoda Edmann concluded that the individuality differential of the 

 embryonal skin had been changed through cultivation in vitro. Mammalian 

 tissue, on the other hand, which had also been explanted in a first period, 

 was soon absorbed after homoiotransplantation. Gassul, in continuing these 

 experiments, noted that if skin of an adult frog, which in tissue culture can 

 remain alive for as long as six weeks, is kept for several weeks in vitro in 

 frog plasma, it continues to live after subsequent homoiotransplantation into 

 the skin of another frog for longer than thirty days, and during this period 

 behaves like an autotransplant, no reaction developing around it. Conversely, 

 if a piece of frog skin has been kept for some time in vitro in foreign serum 



