Primary hypoblast development in the chick : II. The role of cell division

Proliferative patterns in colchicine treated embryos during the first few hours of incubation, Eyal-Giladi and Kochav stages XI-XIII, clarify the role of cell division in the anterolateral expansion of the primary hypoblast as well as the expansion of the entire blastoderm. A layer of primary hypoblast (extra-embryonic endoderm) can form in the absence of cell division. Colchicine, in concentrations used here, does not prevent ingression of cells from the upper layer to the primary hypoblast and establishment of intercellular contacts. These processes, however modified, remain sufficiently operative so that a hypoblast layer forms. In addition, colchicine does not prevent radial expansion of the whole blastoderm. The demonstrated distribution of proliferative rates in the blastoderm can be the source of cells for blastoderm radial expansion, shape changes and for hypoblast development supplementing ingression of cells from the upper layer.