Origin of the hypoblast in Salmo. I. Does the blastodisc edge turn inward?

Nile blue sulfate stain spots printed upon the teleost blastodisc may penetrate to deep cells. Spots printed upon the periblast just outside the rim of the early blastodisc remain stationary, and when overridden by the spreading disc may transmit their stain to cells of its floor that come progressively in contact with them. Such spots do not give reliable evidence of invagination movements, or of a wheeling‐in of cells that lie inside the cellular envelope though close to the surface. Small spots printed upon the surface at the margin of the blastodisc do not necessarily disappear from the margin, and if they do they cannot subsequently be discovered in the interior. Patterns of chalk particles injected into the blastodisc for the purpose of demonstrating the origin of the hypoblast by a wheeling‐in movement of cells at the rim have produced no such evidence, but show a direct migration of originally internal cells toward the definitive somites. Conclusions drawn from the Salmo fate map and scheme of morphogenetic movements (Pasteels, '36) are not substantiated insofar as they pertain to the origin of the hypoblast and the behavior of cells at the blastodisc rim. There is no inturning of cells to produce the hypoblast, either at the axis or along the germ ring.