The explanation of the wide-spread changes in the embryo after grafting with adult organs offers a problem of considerable interest. Just what factors are responsible for them we are unprepared to say. They occur only after the grafting of certain adult homologous organs and are absent after grafting with such tissues as the chicken sarcoma, adult muscles, bone, and cartilage. Likewise no systemic changes have been observed after grafting of various tissues from foreign species, such as rat and mouse embryos, rat and mouse tumors of various sorts, or the normal organs of an adult animal of a foreign species. Whether or not this extensive proliferation of the white blood cell elements is to be looked on as a stimulation of a center with metastasis in the membranes and organs, or whether it is a general stimulation to the anlage of the various cells, is a point difficult to determine. Judging from the distribution and character of the proliferation, the latter seems the more plausible view. Moreover, the reaction may be of the same type as that observed by Da Fano in mice after immunization against cancer by means of tissue injections. The changes he describes, however, were not so pronounced in character and were for the most part confined to the plasma cell in the connective tissues. The possibility of the action of infection must also be considered. Smears from the embryo spleen and the scattered nodules failed, however, to show microorganisms, and cultures taken on the ordinary media were negative for bacteria. If the reaction was a result of bacterial invasion, we should expect to find the changes occurring after the implantation of some of the transplantable rat and mouse tumors, for it is well known that they become highly contaminated. But such is not the case. A possible explanation of the necrosis is that the nutrition supply does not keep pace with the growth, so that a condition develops analogous to that of the central necrosis in rapidly growing tumors. We conclude, therefore, that grafts of adult spleen, bone marrow, liver, and kidney placed in the outer membrane of a chick embryo cause stimulation of the embryo spleen and lead to proliferation of certain leukocytic elements in the mesoderm, subcutaneous tissues, and around vessels in the liver and kidney.

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