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A synthetic medium suitable for the growth of tissue cells outside of the animal organism has not been discovered up to the present time. Since the preparation of such a medium would lead directly to a better understanding of cellular metabolism, this problem has stood forth as one of the most important of those presented by the tissue culture method.From earlier observations evidence had already been obtained which showed that the ingredients essential for the building of new cells and the liberation of energy in the cultures comes directly from the tissue fragments. The growth observed is a manifestation of a simple transfer of materials from the more central portions of the fragment to the cells at the periphery, or, in other words, the preying of one cell in a more suitable environment upon its neighboring cel]s in an unsuitable one. This is true in cultures where simple isotonic salt solutions have been used as the medium. That it is also true in the case of the plasma culture can be readily shown by repeatedly changing the medium or transplanting the cells to drops of fresh medium. All activity ceases after a few transplants, or when the cells within the fragment have become exhausted.One of us# in a recelit study, has shown that tissue cells are not highly organized elements. Their various manifestations of life are the result of reactions between themselves and a specifically