It has already been shown (1933, 1936) that chick fibroblasts (mesenchyme cells derived from the heart or from periosteal tissue) can be reduced to a state in which growth by cell division is almost at a standstill and that, by the addition to the medium of embryo tissue juice, the cells can be caused to divide again. The higher the concentration of embryo juice used, the greater is the number of cell divisions which occur. The addition of the embryo juice is followed within the first 2 hours by increased movement of the cells, which reaches its maximum in about 10 hours, but it is not until this latter time that cell divisions begin again.

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