Primary hypoblast development in the chick : I. Scanning electron microscopy of normal development.

Scanning electron microscopy (SEM) indicates that the primary hypoblast forms beneath the area pellucida during the first 8 h of incubation mainly by establishment of contact among cells which move downward out of the epiblast. This movement, polyingression, begins posteriorly and continues antero-laterally during the period of primary hypoblast formation. Polyingression produces many pits and possibly a crescentic fold in the embryo upper surface with corresponding cell clusters and a ridge on the lower surface. Fixationin situ helps prevent formation of artifactual folds and wrinkles facilitating interpretation of the SEM images.Formation of intercellular adhesions which lead to development of an epithelial primary hypoblast proceeds in a posterior to anterior direction along with polyingression. This epithelialization begins with elaboration of numerous filamentous processes by cells as they arrive from the epiblast, and continues with ongoing input of cells, merging of cells and cell clusters, and cell flattening. We have also shown (Weinberger and Brick 1982) that proliferation of ingressing cells provides additional cells for hypoblast development.