The chick embryo late primitive streak and head process studied by scanning electron microscopy.

Changes in cell shape during the formation of the head process and regression of Hensen's node have been examined by scanning electron microscopy in a series of chick embryos from stage -5 to stage -8. The endoderm was removed from embryos mounted for New culture so that the mesoderm could be viewed from the ventral surface. As the primitive streak shortens its cells flatten and lose the flap-like processes characteristic of earlier stages. The notochord differentiates and its cells become transversely orientated, then re-align along the embryonic axis between stage 6 and stage -8. At the same time a split or cleft appears between notochord and head mesoderm, and extracellular fibrils are formed which are the first sign of the formation of a notochordal sheath.