178 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



As mentioned already, it is only at a certain stage in the embryonic develop- 

 ment that organismal differentials are fully developed, and there is every 

 reason to assume that the development of the mechanisms which are directed 

 against strange organismal differentials takes place only subsequent to the 

 complete formation of the organismal, and, in particular, of the individual- 

 ity differentials. As we shall discuss more fully later, even in the very 

 young guinea pig not long after birth the reactions against homoiogenous 

 tissues are weaker than in older animals. The experiments of Murphy suggest 

 very strongly that the spleen and the reticulo-endothelial system in general are 

 the tissues which originate these mechanisms of defense, or at least aid in 

 their development. We have furthermore seen that the lymphocytes are at- 

 tracted by strange individuality differentials and that they may help in the 

 destruction of homoiogenous tissues ; but it is possible that also the individu- 

 ality and species differentials, which are attached to certain substances in the 

 bodyfluids and which exert a primary toxic effect on homoiogenous and 

 heterogenous tissues, may develop with the aid of the reticulo-endothelial 

 system. These considerations concerning the lack of individuality differentials 

 and of the mechanisms of reaction against the latter in early embryos, would 

 then explain why the chorio-allantois of the chick and, according to Murphy, 

 also the chick embryo as such, do not oppose the preservation of heterogenous, 

 actively-growing tissues, such as malignant tumors and embryonal tissues. 

 According to Taylor, Thacker and Pennington, it seems that mammalian 

 tumors grow very well also in the yolk sac of the chick embryo. 



There are also, in adult animals, at least two sites where heterogenous 

 tumor transplants may survive, namely, the brain and the anterior chamber of 

 the eye. Shirai found that heterogenous tumors, which cannot be successfully 

 transplanted elsewhere, may grow when transplanted into the brain. Murphy, 

 who obtained similar results, observed that around such heterogenous trans- 

 plants in the brain the usual lymphocytic reaction is absent; but, as in the 

 case of transplantation into the allantois, a lymphocytic reaction can be called 

 forth if simultaneously with the grafting of the tumor a piece of spleen is 

 transplanted into the brain. However, when Harde transplanted homoiogenous 

 tumors to the brain, no differences in results between the brain and the usual 

 sites of transplantations were noted. Siebert compared with the reactions 

 against tumor tissue, those against homoiogenous thyroid transplants in the 

 guinea pig, the time of the examination of the graft varying between 20 and 

 120 days after transplantation. He found the amount of homoiogenous thyroid 

 gland that was preserved in the brain less than that of autogenous transplants. 

 Much fibrous-hyaline tissue developed in or around the homoiogenous graft. 

 After 20 to 30 days, only a few small acini were preserved in the hyaline 

 stroma. Lymphocytic infiltration appeared in the fibrous tissue invading the 

 transplant and some scattered lymphocytes also surrounded the brain tissue, 

 but on the whole, the lymphocytic infiltration was much less intense than after 

 transplantation into subcutaneous pockets. The homoiogenous thyroid tissue 

 remained longer preserved in the brain than it is usually in the subcutaneous 



