Using a new technique to visualize the tracks of moving 3T3 cells and combining it with the visualization of actin-containing microfilament bundles by indirect immunofluorescence (Lazarides, E. and K. Weber. 1974, Proc. Natl. Acad. Sci. U.S.A. 71:2268-2272), I present experiments which suggest that: (a) 30-40% of the pairs of daughter 3T3 mouse fibroblasts in noncloned cultures have mirror symmetrical actin-bundle patterns. (b) The angle between separating daughter cells is approx. 90 degrees or 180 degrees and seems related to the directions of certain actin-containing bundles. (c) Approximately 40% of separately moving daughter cells which did not collide with any other cell in the culture performed directional changes in a mirror symmetrical way. Both daughter cells entered the next mitosis at approximately the same time. I suggest that the actin-bundle pattern, the angle of separation, major directional changes during interphase, and the time of the next mitosis are predetermined by the parental cell.

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