“…Figure 4 shows that in a checkerboard consisting of y-irradiated monolayers of BMS, Cl4, CZl, and C23 cells (rows) overlaid with no cells or lo4 BMS,C14,C21,or C23 cells (columns), all cell types could form colonies on all others, except for BMS, C14, and C23 cells on C23 monolayers. The reasons for such selective growth abilities are unknown, hut several similar instances have been reported from which no clear pattern has emerged (Stoker, 1964;Ponten and MacIntyre, 1968;Eagle et al, 1968;Weiss, 1970;Pletyushkina et al, 1975;Sakiyama et al, 1978).…”