The overall objective of the research plan is to further elucidate the role of tissue interactions in the development of the cranio-facial region of the vertebrate using the mouse and chick embryos as model systems. Within this overall objective, two major questions are addressed: 1) what are the tissue interactions involved in the events preceding and leading to the programmed cell death of medial epithelium in the mammalian palate, and 2) what are the interactions which direct the differentiation of cartilageand membrane bone from cranial neural crest cells within the facial region. Studies will be made at the light-microscopic and ultrastructural levels on the results of tissue isolation, recombination, and graft experiments. The results are expected to be important to the understanding of normal development of the secondary palate and skeletal elements of the facial region, and to give information concerning the causes of cleft palate formation and facial skeletal anomalies.