582 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



or plasma and is then homoiotransplanted into a living host, it is cast off 

 after two to five days. Gassul assumed that cultivation in a heterogenous 

 medium altered the tissue in such a way that it assumed a heterogenous 

 character. As far as homoiotransplantation is concerned, he concluded that 

 the preceding explantation in a homoiogenous medium caused an enhance- 

 ment of the individuality of the transplanted tissue. However, the success 

 of the homoiotransplantation under these conditions might rather be inter- 

 preted as signifying that the individuality differential of the transplant, or, 

 rather, the intensity of its production, has been weakened, and it elicited 

 therefore a weaker reaction on the part of the host. Furthermore, Gassul's 

 conclusions are based on a very small number of experiments in which 

 tissues were homoioiransplanted and only some of these successfully. As to 

 the action of heterogenous plasma, especially that of warm-blooded animals, 

 on tissues kept in vitro, this procedure seems to diminish the success of a 

 subsequent homoiotransplantation by causing injury of the tissues. 



Subsequently, Rhoda Erdmann undertook heterotransplantation of anuran 

 skin after a preceding cultivation in vitro, in an extensive series of experi- 

 ments. While, according to the author, normal adult skin of urodeles can be 

 readily transplanted to other urodele species, it is very difficult to accomplish 

 such a result in anuran species. The first of these experiments concerned the 

 transplantation of skin of Rana esculenta to Rana temporaria, and vice versa. 

 Later, pieces of skin from farther distant species were used for grafting, 

 following a previous cultivation in vitro. By means of this procedure an 

 exchange of skin between different families was made to succeed to some 

 extent ; thus, skin of Buf o and Bombinator could be transplanted to Rana 

 esculenta, which proved to be the most suitable heterogenous host. Bufo skin 

 was first cultivated in a mixture of Bufo plasma and Bufo spleen extract, 

 then in a combination of Bufo plasma and frog extract, and at last in a 

 mixture of frog plasma and frog extract. Skin thus prepared and afterwards 

 transplanted into adult Rana esculenta was found living and united with the 

 skin of the host even as late as fifty days, and not only the transplanted 

 epidermis but also the skin glands of Bufo survived under these conditions. 

 In the case of the skin of Pelobates, cultivation in vitro for a period of 

 twenty-four days was required before it could be successfully transplanted 

 into Rana esculenta. It is of interest that in some experiments of hetero- 

 transplantation into farther distant hosts, hemorrhage killed the host, owing 

 apparently to toxic effects exerted by the transplant. 



In interpreting these results we have to consider several possibilities: 

 (a) In vitro, skin undergoes regenerative growth. It is conceivable that 

 transplantation of regenerating skin gives better results than transplantation 

 of ordinary resting skin although in our earlier experiments we did not 

 observe that homoiotransplantation of regenerating skin in the guinea pig 

 differed essentially from homoiotransplantation of normal skin, (b) If skin 

 is cultivated in vitro, it is only the epidermis that is active and grows, while 

 the underlying connective tissue remains inactive and may become detached 

 from the overlying epithelium. Thus after transplantation of this tissue, the 



