Intact mandibular processes and the enzymatically separated mesenchymal and epithelial components of the mandible from embryonic chicks of 2.5- to 5-day incubation (Hamburger and Hamilton, '51: stages 16-25) were grown individually, either in organ culture or as grafts to the chorioallantoic membranes of host embryos. The differentiation of cultured and grafted intact mandibular processes was histologically normal, but the time of histodifferentiation differed from that in vivo. The histodifferentiation of cultured and grafted mandibular mesenchyme grown isolated from its epithelium depended upon the age of the embryo from which the mesenchyme had been obtained. Intramembranous ossification producing membrane bones of the mandible occurred in mesenchyme isolated from 4.5- to 5-day embryos (HH 24–25), but did not occur in mesenchyme isolated from younger embryos. Cartilage (Meckel's) and subperichondrial bone in the articular process of Meckel's cartilage differentiated in mesenchyme isolated from embryos of all age groups tested (HH 16–25). Mandibular mesenchyme, therefore, requires the presence of epithelium until 4.5 days of incubation if the membrane bones of the mandible are to differentiate; if epithelial influences are required for Meckel's cartilage and subperichondrial bone formation, they are not required beyond 2.5 days of incubation. Mandibular epithelium isolated from its mesenchyme became layers of squamous cells in culture; but when grafted onto the chorioallantoic membrane, the epithelium became underlain by host fibroblasts and differentiated into a stratified squamous epithelium. Mandibular epithelium, therefore, is capable of differentiation in the presence of foreign fibroblasts derived from the chorioallantoic membrane.