The present paper deals with a comparison of the conditions which determine the fate of transplanted tumor and of a transplanted normal tissue capable of growth. Mouse embryo and mouse-tumor were employed as material. It was shown that individuals differ as hosts for transplanted embryo, some being naturally resistant to its growth, and some favorable, just as is known to be the case where tumor is concerned. The fate of implanted tumor depends directly on whether it elicits from the host a vascularizing stroma. So, too, it is with implanted embryo. Furthermore mouse-embryo, like mouse-tumor, when introduced into rats calls forth a stroma and grows for a brief period. In attempt to answer the question as to whether individuals favorable (or resistant) to implanted tumor are likewise favorable (or resistant) to implanted embryo, it was shown that the factors of age, nutritive condition, and race, which are potent in determining an animal's status as a tumor-host, act similarly in determining that for embryo. Using embryonic tissue and a method which has proven effective for the production of immunity to implanted tumor, an immunity to implanted embryo was brought about. This immunity manifests itself in the same way as that for implanted tumor, namely, by an absence of the stroma-reaction necessary to life of the engrafted tissue. These results demonstrate how largely tumor obeys in its adaptation to a new host and growth therein, the general laws regulating a transplanted normal tissue. Besides the phenomena here dealt with many others that have held the attention of workers with transplantable tumors are probably not peculiar to neoplasm. The present findings emphasize the importance of the tumor-problem as a tissue-problem; and they further indicate how essential it is in cancer work to discriminate between characters unique with tumor and those which it possesses in common with normal tissue.