Rat tumors and other tissues of foreign species grow actively in the chick embryo until the onset of a refractory period. Grafts of rat sarcoma established and growing actively at the onset of this period show a rapid cessation of growth between the 18th and 19th days of incubation. This is followed by a widespread degeneration of the rat cells and a marked activity of the connective tissue elements in the embryonal tissue round about. An occasional specimen may show small mononuclear infiltration or rarely accumulations of polymorphonuclear cells in the neighborhood of the strange tissue. During the 20th and 21st days the connective tissue capsule increases rapidly, invading the graft and replacing it. The foreign cells disintegrate rapidly and by the 22d day have practically all disappeared. The period at which the established graft begins to degenerate, namely the 19th to the 20th day of incubation, is the one on which grafts of foreign tissue will no longer take when implanted in the embryo. The cells at the edge of the graft of foreign tissue survive for a time. There is a rapid formation of a connective tissue capsule and an invasion of the grafts. The foreign cells here practically all have disappeared by the 22d day, leaving a mass of connective tissue. The absence of a round cell infiltration is the most marked difference in the process in the embryo as compared with that in the adult chicken. The process about the grafts in newly hatched chicks is characterized by a more active response of the connective tissue than in the adult and a more pronounced round cell infiltration than in the embryo.

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