The cells of a mouse mammary cancer were obtained with enzymes in suspensions which could be filtered to exclude all which were not single and most which were already dead.

Heavy suspensions of these individual cells were plated over the dorsal subcutaneous expanses of female weanling mice where they implanted and grew to form coalescent tumors covering the back more or less entirely.

Sparser suspensions, similarly plated (a) gave rise to fewer tumors, and (b) gave rise to tumors reaching measurable size later.

These two consequences of sparser plating left room for the testing of adjuvators to transplantation.

Adjuvator effects were obtained by splitting the subcutaneous expanses beforehand and by injecting liver along with the plated cells.

Through 28 plated generations over 4 yr the tumor maintained completely stable, morphological heterogeneity.

These findings with a complex tumor indicate strongly that its heterogeneity comes not from repeated cellular mutation late in its development, but from diverse potentiality to give rise to cells of specific and differing character, inherent in its individual cells at the time of its multicellular origin.

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