Our research concerns the migration and differentiation of the neural crest cells. We have used for this investigation a cell marking technique devised by the chief investigator of this proposal and based on structural differences between the interphase nucleus of two species of birds: the chick (Gallus gallus) and the quail (Coturnix coturnix japonica). Special emphasis has been done this year on the origin of the endocrine cells of the gut epithelium and of the pancreas from the neural crest. According to Pearse's theory of the "APUD-series", those cells, along with the adrenomedullary cells, and the calcitonin producing cells (C-cells) of the ultimobranchial bodies, would have a common ectodermal origin. In fact, large series of experiments allowed us to consider that the C-cells are derived from the rhombencephalon, but that the endocrine cells associated with the gut originate neither from the neural crest nor from the neurectoderm. We conclude that their origin from the endoderm is the most likely hypothesis at the present time. The second point investigated concerns the determination of NC cells and the influences that the microenvironment exerts on their differentiation. We found that, as far at the autonomic nervous system is concerned, the biochemical differentiation of the autonomic ganglioblasts (in terms of neurotransmitter sysnthesis) depends upon environmental influences which are dictated to the cells by the mesenchyme in which they migrate. In addition, the phenotypic expression of the autonomic ganglioblasts remains labile a certain time after their differentiation has been initiated.