The results of autoplastic and homoplastic transplants of PET mouse tissues indicate that the state of differentiation of the developing muscles limits the extent of invasion by pigment cells of the muscles of the leg in PET mice. Additionally, intracoelomic grafts into host chick embryos of PET mouse leg tissues has given evidence which suggests that the pigment cell behavior is affected also by a restriction of pigment cell proliferation and concurrent migration by the initiation of pigment cell morphogenesis. Thus it appears that the colonization of the leg muscles by pigment cells in the PET mouse is governed both by the structural limitations and morphogenetic influence imposed upon the melanoblasts by the tissue substrate.