The intact 11-day dorsal pancreatic rudiment, cultured in vitro, develops to a stage of differentiation roughly corresponding to the pancreas of an embryo of 15 days. Epithelium cultured alone does not develop. Epithelium recombined with pancreatic mesenchyme, or with a number of heterogenous mesenchymes, undergoes morphogenesis resembling that of the intact rudiment. Though the detailed pattern of lobulation varies for each mesenchyme used, differentiation at cell and tissue levels was never anything other than pancreatic in type. In transfilter experiments epithelial growth and aberrant morphogenesis were promoted strongly by salivary mesenchyme, to somewhat lesser degree by pancreatic mesenchyme, and little or not at all by metanephrogenic mesenchyme.