THE differentiation into muscle elements of cells originating in explants of embryonic muscle tissue has been repeatedly studied. The literature has recently been reviewed by Godman [2] and individual references will not be given here. In the case to be described differentiation was both constant and rapid and the method may be suitable for studies of specific aspects of the phenomenon of differentiation. Differentiation was observed in monolayer cultures of chick embryo cells, grown in sealed plastic dishes [l] and overlain with nutrient agar. However, they could equally well be grown in glass petri dishes in an atmosphere of 5 per cent carbondioxide. The monolayers were prepared by a method which has become standard laboratory practice. A cell suspension was prepared by maceration and trypsin digestion of chick embryos, aged 11 to 13 days, from which the eyes and claws had been removed. Digestion was carried out at 37°C using a solution of 0.25 per cent trypsin in phosphatebuffered saline. Each dish contained an initial inoculum of 5.105 to 106 cells in 4 ml of growth medium consisting of 4 parts horse serum, 5 parts Earle’s saline, and 1 part calf-embryo extract with penicillin and streptomycin. After incubation for 48 hours at 37°C the cells had grown to produce a monolayer consisting largely of fibroblast-