The formation of the endoderm during primitive streak stages in avian embryos was studied by combining several of the following techniques for each embryo. These included microsurgery, time-lapse filming, use of chick-quail chimaeras, tritiated thymidine autoradiography and a novel technique for identifying the morphology of the cells after small pieces of tissue from known areas had been maintained in culture for 24 h. Using these techniques we have confirmed that the ventral layer of the early chick embryo receives contributions from both the marginal and the central regions of the area pellucida. The former seems to consist of yolky cells derived from the germ wall, whilst the latter consists of smaller, less yolky cells derived from the more dorsal layers of the embryo. The movement of the lower layer anteriorly during these stages appears to be dependent upon mechanical constraints imposed upon it by the expanding tissue in more caudal regions. The extent of each of the two contributions to the lower layer was determined as a function of stage and presence or absence of a lower layer, and the findings are discussed in the light of the existing literature.