Origin of the hypoblast in Salmo. II. Outward movement of deep central cells

Vital dye carriers implanted under the blastodisc and chalk markers injected among the blastomeres provide a new interpretation of the origin of the hypoblast in teleost embryos. This first inner layer of cells is formed along the sides of the embryonic area and around the germ ring, not by invagination from the surface, or by wheeling in of deeper cells, or by delamination at the blastodisc rim, but by the migration outward of deep central cells. The layering is interpreted as the effect of a shear between superficial cells in pure epibolic motion and deeper cells which are at the same time undergoing convergent movement toward the axis. The whole blastodisc could be stained to controllable depths by suitable exposure to vital dye solutions. Subsequent location of the colored cells indicates that at the late “pregastrula” stage the prospective CNS cells overlap those which will become the first 10 pairs of somites, and all the prospective somite material overlaps prospective notochord and endoderm. A teleost pregastrular fate map thus cannot be a surface map as in amphibia. The rim of the teleost blastodisc and the amphibian blastopore carry out totally different functions.