1. It is shown that evidence similar to that which makes probable the potential immortality of protozoa and germ cells also exists in the case of somatic cells of metazoa. As far as the protozoa are concerned, the discussion does not, as seems to be assumed, concern their potential immortality so much as the question whether protozoa correspond to germ or to somatic cells of metazoa or represent perhaps a combination of both. 2. While in the case of tumor cells the potential immortality of somatic cells has been demonstrated as definitely as the character of the problem will ever permit, the difffientries standing in the way of a similar demonstration in the case of certain other somatic cells and the means of overcoming these difficulties are analyzed. It is particularly shown that chemical differences existing between the body fluids of individuals belonging to the same species are the basis of these difficulties, and that as a result of these differences the metabolism of cells in a new environment is ...