The behavior of individual fibroblasts in vitro was described in a previous paper (1). It was observed that single, isolated fibroblasts never divided or multiplied, and that fragments of cultures of fibroblasts with few and scattered individuals were not able to multiply in vitro. This fact suggested that direct protoplasmic connections or anastomosis with a certain number of homologous cells of the same species were significant in some way for the initiation of cell division.