186 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



As to the reason why transplantations from very young donors may succeed 

 better than those from older ones, our knowledge is still less definite. How- 

 ever, by means of the white blood cell reaction Blumenthal could show that 

 both tissues from early as well as from later stages of rat embryos elicited a 

 lymphocytic reaction after transplantation into adult rats; but rat or mouse 

 embryos obtained during the first half of pregnancy called forth, in a 

 heterogenous host, merely an increase in lymphocytes, as an indication that 

 the organismal differentials were not yet fully formed at this period ; a short 

 time before the end of pregnancy the typical heterogenous reaction did 

 develop. In this connection, the fact must be recalled that also implantation 

 of non-living protein substances, may call forth a lymphocytic reaction and 

 it is therefore possible that a non-specific or at least a less specific, factor 

 caused the effect which very young embryonic tissue exerted on the lympho- 

 cytes of the host. It may then be concluded from these experiments that the 

 organismal differentials are fully developed in newborn animals, and if tissues 

 from very young donors survive better in homoiogenous hosts than those 

 from older ones, this must be due to other factors than lack of differentials. 

 In this regard we have to consider, in the first place, the greater growth 

 momentum of the younger tissues, and perhaps also their greater adaptability 

 to inadequate environmental conditions. The increased growth momentum 

 may be able to overcome injurious conditions, which more slowly growing, 

 adult tissues cannot overcome so readily. This view would be in harmony with 

 the experience that transplanted rapidly growing tumors which possess a 

 strong growth momentum may be more resistant to the action of unfavorable 

 individuality differentials than normal tissues, and, similarly, embryonic 

 tissue may be more independent of the action of such differentials. 



There is still a further point to be considered. Certain organs from old 

 animals show changes which make them less suited for transplantation than 

 the corresponding organs from younger ones; thus the ovaries of older mice 

 and of other species contain few follicles, and the thyroid gland in certain 

 strains of mice undergoes sclerosis ; these are conditions not favorable to a 

 good development and function of the essential constituents of the organs 

 when transplanted. 



We have attempted in this analysis to separate the various factors which 

 may cause the difference in the results in carrying out transplantation experi- 

 ments, using young and old animals as hosts and donors, and we have found 

 that the individuality differentials are fully developed in young donors and 

 that a lack of the differentials is not one of the factors that causes the 

 difference in the results of homoiotransplantation in animals of different 

 ages. The greater tendency to the formation of fibrous tissue in older, 

 homoiogenous hosts and the greater growth energy of younger tissues may 

 explain at least some of these differences. 



