Neural crest, or regions containing presumptive neural crest, from chick embryos between late primitive streak stage and 20 somites, was explanted in plasma clot cultures on depression slides, and grown for periods ranging from 3 to 14 days. 2. During the first four days in vitro, differentiation of pigment occurs, the colorless refractile granules of the cells gradually deepening in tone from yellow to deep brown or black, until large numbers of typical melanophores are present. 3. The most intense pigmentation occurred in cultures from embryos of dominant black breeds. There is an apparent positive correlation between the presence of the genetic factor for black and the capacity for pigment production, since hybrids carrying the black factor, and dominant white in which the presence of the black factor is masked by an inhibitor, also produce pigment in cultures of neural crest, while cultures of recessive white which lack the genetic factor for pigment were negative. 4. Pigment production is related to the stage of the embryo at the time of explantation. A very low percentage of positives was obtained from embryos prior to the formation of the neural folds, the highest percentage of strongly positive cultures being obtained from embryos of from one to 7 somites, when the neural crest material is most concentrated. The lower percentages obtained in early stages are probably due to the undetermined nature of the explant, in the older stages to the increasing diffuseness of the neural crest as a result of cellular migration. 5. Control cultures, made from other regions of the embryo, were invariably negative when taken in stages prior to migration of the neural crest, except in cases where the whole embryo was explanted after removal of the pre-otic crest. Here a few black cells occurred as probable derivatives of the post-otic section of the crest.
