Just prior to streak formation (stage XIII) the 2-layered chick blastoderm is formed by the 1-layer epiblast which needs the influence of the hypoblastic layer to develop an embryonic axis. To examine the differentiation capacities of these cells in culture and to compare them with hypoblastic cells, a study was made of this latest possible stage in the development of the chick in which 1 cell population, the epiblast, is still totipotential. In studying differentiation, heterogeneity of the starting cell population was minimized by culturing either hypoblastic cells or epiblastic cells. The epiblastic cells were derived from epiblasts deprived of the marginal zone and of the area opaca. Hypoblastic cells formed a 1-cell thick characteristic epithelium. Epiblastic cells in culture evolved from a homogenous sheet to clearly demarcated ares to dome structures which resemble embryoid bodies from teratocarcinomas. Histologically, 3 main tissue types were found in the epiblastic cultures. Sometimes, the borderline between 2 of the tissue types was clearly demarcated by a basement membrane. Both hypoblastic and epiglastic cells produced a basement membrane-like structure when cultured in vitro. The appearance of mesoderm in the epiblastic cultures was particularly interesting and it was evident by the appearance of blood islands and clearly defined endothelial-lined cavities. No complex organized embryonic structures of any kind were found in the cultures.
