2. It is practically impossible to transplant organs or tissues from one species of higher animals to another, unless the two species are very closely related; and even then the transplantation is uncertain and the graft may either fall off again or be destroyed. This specificity of tissues goes so far that surgeons prefer, when a transplantation of skin in the human is intended, to use skin of the patient or of close blood relations. The reason why the tissues of a foreign species in warm-blooded animals cannot grow well on a given host has been explained by the remarkable experiments of James B. Murphy of the Rockefeller Institute.[32] Murphy discovered that it is possible to transplant successfully any kind of foreign tissue upon the early embryo of the chick. Even human tissue transplanted upon the chick embryo will grow rapidly. This shows that at this early stage the chick embryo does not yet react against foreign tissue. This lack of reaction lasts until about the twenty-first day in the life of the embryo; then the growth of the graft not only ceases but the graft itself falls off or is destroyed. Murphy noticed that this critical period coincides with the development of the spleen and of lymphatic tissue in the chick and that a certain type of migrating cells, the so-called lymphocytes, which develop in the lymphatic tissue, gather at the edge of the graft in great numbers, and he suggested that these lymphocytes (by a secretion of some substance?) rid the host of the graft. He applied two tests both of which confirmed this idea. First he showed that when small fragments of the spleen of an adult chicken are transplanted into the embryo the latter loses its tolerance for foreign grafts. The second proof is still more interesting. It was known that by treatment with Roentgen rays the lymphocytes in an animal could be destroyed. It was to be expected that an animal so treated would have lost its specific resistance to foreign tissues. Murphy found that this was actually the case. On fully grown rats in which the lymphocytes had been destroyed by X-rays (as ascertained by blood counts) tissues of foreign species grew perfectly well. These experiments have assumed a great practical importance since they can also be applied to the immunization of an animal against transplanted cancer of its own species. Murphy found that by increasing the number of lymphocytes in an animal (which can be accomplished by a mild treatment with X-rays) the immunity against foreign grafts as well as against cancer from the same species can be increased. It is quite possible that the apparent immunity to a transplantation of cancer produced by Jensen, Leo Loeb, and Ehrlich and Apolant through the previous transplantation of tissue in such an animal was due to the fact that this previous tissue transplantation led to an increase in the number of lymphocytes in the animal. The medical side, however, lies outside of our discussion, and