1. Histological studies show that the mechanism of colchicine inhibition of regeneration in pieces of Dugesia dorotocephala is the stathmokinetic action it exerts on free parenchymal amoebocytes. 2. Parenchymal amoebocytes are the only cells exhibiting mitotic activity during the period of regeneration. 3. Mitotic activity reaches a peak at the third day of development while oriented migrations of amoebocytes appear to set in at the second day with marked movement through the fourth to the sixth day after section. 4. The free amoebocytes of the parenchyma constitute the exclusive source of cells participating in the replacement of parts lost when planarian worms are sectioned.