The morphogenetic movements at the lower surface of the blastodisc in salmonid embryos

Abstract

AbstractPatterns of hitherto undescribed morphogenetic movements have been found in Salmo gairdneri and Salvelinus fontinalis by removing samples of blastodiscs from their yolk spheres at intervals of several hours and inspecting their lower surfaces. Before onset of epiboly, the former species usually produces no pregastrular subgerminal cavity, but the latter usually produces one or several, with wide variation. In both species the inner blastomeres consolidate from an earlier dispersed and migratory condition, forming a coherent flattened sheet 6 to 10 cells thick before active epiboly begins.As the first evidence of polarity in the salmomd embryo, one sector of the blastodisc becomes condensed and slightly thickened. Then a broad central area of the disc rapidly thins out as a result of two types of movement: (1) large numbers of cells disengage from the under surface of the condensed blastodisc, and they migrate away from the center as individuals and small groups; (2) the rest of the internal cells engage in a similar centrifugal movement toward the germ ring and the embryonic shield, but as a coherent sheet, sliding just within the cellular envelope.Some, perhaps all, of the disengaged cells of the first group are slowly gathered into the endoderm sheet. The coherent cells of the second group accumulate in germ ring and embryonic shield, where they become split temporarily into upper and lower sheets, the former adhering to the external layer, the cellular envelope, with which it forms the epiblast, and the latter associating with the disengaged cells as the hypoblast.The first internal sign of axis formation is a hitherto undescribed nubbin of cells attaching to the yolk syncytium at one sector on the inside of the blastodisc rim. The nubbin cells take no further part in the epibolic spread of the disc. It is suggested that they from an anchor for the later convergence movements from right and left, and that the whole pattern of movements from then on may be controlled by a gradient of increasing cell adhesiveness that spreads from them as a center. These nubbin cells have never been at the surface of the blastodisc. They neither invaginate nor migrate forward. They may represent the prechordal plate.