254 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



always cartilage, and, to a lesser extent, bone, osteoid tissue and bone mar- 

 row, and perhaps also smooth muscle tissue, squamous epithelium with its 

 appendages, the hair follicles and sebaceous glands, which have the best 

 chance to survive; to a lesser extent, entodermal (intestinal) and lymphoid 

 tissue, and much more rarely, nerve tissue and rudimentary formations of 

 sense organs, as well as glands such as liver, may be found in these embryo- 

 mata. 



Mammalian embryonal tissues give evidence of possessing species differen- 

 tials and, consequently, do not withstand much better than adult tissues the 

 injurious effects of heterotransplantation. Thus Rous observed that embryonal 

 tissue of the mouse may grow in the rat for a few days, but then it dies. 

 However, as to the individuality differential, there seems to be less selective- 

 ness here, after homoiotransplantation, than in adult tissues ; but this may be 

 at least partly due to the fact that the original growth momenium is greater 

 in embryonal than in adult tisues, and hence the former may be carried 

 over some of the difficulties to which the latter succumb. Yet we find, also, 

 in transplantation of embryonal tissues, great differences in the results 

 obtained in different hosts of the same species. This indicates that individuality 

 differentials may, after all, play a certain role in determining the fate likewise 

 of the embryonal grafts. But in transplantation of mammalian tissue the 

 age and developmental stage of the embryo, too, may be of some importance 

 in the differentiation of the organismal differentials; in very early stages 

 even species differentials are not yet fully developed. 



That in transplantations of both normal adult and embryonal tissues the 

 genetic relationship between transplant and host is a very essential factor in 

 determining the outcome is further indicated by the fact that lymphocytic 

 infiltration takes place around grafted living embryonal tissue. Two weeks 

 after transplantation of embryonal tissue into the stomach wall of rats, 

 Askanazy found an accumulation of lymphocytes around the transplant, and 

 W. P. Neilson noted the same occurrence more recently in our laboratory. 

 However, the interpretation of these observations is complicated by the 

 fact that transplanted embryonal tissue not only grows, but also differentiates, 

 and thus in the course of time becomes more like an adult tissue, and we 

 cannot therefore be certain how much the maturation of the embryonal 

 tissues had to do with the accumulation of lymphocytes. 



Rous, in making two grafts of the same embryonal tissue into different 

 places in the same host, observed that both transplants behaved in the same 

 manner ; either both did well or both retrogressed at an early date, or neither 

 took, a finding analogous to ours in the case of tumors. This might be in- 

 terpreted as indicating the importance of the individuality differentials of 

 host and transplants in determining the result. But, Rous also found that 

 mouse tumor tissue and mouse embryonal tissue transplanted simultaneously 

 into the same mouse had a similar fate. Inasmuch as the individuality differ- 

 entials of the embryonal and tumor grafts in this case were not identical, 

 it appears that the sensitiveness and intensity of the reaction of a host 

 towards strange individuality differentials in general likewise were determin- 



