we must satisfy ourselves with only a passing notice. The facts show that each warm-blooded animal seems to possess a specificity whereby its lymphocytes destroy transplanted tissue taken from a foreign species. [32] Murphy, J. B., _Jour. Exper. Med._, 1913, xvii., 482; 1914, xix., 181; xix., 513; Murphy and Morton, J. J., _Jour. Exper. Med._, 1915, xxii., 204. A lesser though still marked degree of incompatibility exists also in lower animals for grafts from a different species.[33] The graft may apparently take hold, but only for a few days, if the species are not closely related. Joest apparently succeeded in making a permanent union between the anterior and posterior ends of two species of earthworms, _Lumbricus rubellus_ and _Allolobophora terrestris_. Born and later Harrison healed pieces of tadpoles of different species together. An individual made up of two species _Rana virescens_ and _Rana palustris_ lived a considerable time and went through metamorphosis. Each half regained the characteristic features of the species to which it belonged. It seems, however, that if species of tadpoles of two more distant species are grafted upon each other no lasting graft can be obtained, _e. g._, _Rana esculenta_ and _Bombinator igneus_. These experiments were made at a time when the nature and bearing of the problem of specificity was not yet fully recognized. The rôle of lymphocytes in these cases has never been investigated. The grafted piece always retained the characteristics of the species from which it was taken. [33] The reader is referred to Morgan’s book on _Regeneration_ (New York, 1901), for the literature on this subject.