History of the hypoblast in Salmo

Abstract

Spots printed on the blastodisc o f Salmo gairdneri with Nile blue sulfate prior to and during the formation of the germ ring and embryonic shield show during the morphogenetic movements that not many of the germ ring cells are derived from the center of the blastodisc. Evidence suggests that only a few of the loose round cells of the deep part of the ring and of the axial nubbin are produced by local dropout of cells at the margin of the blastodisc, but that most of them arrive there by outward migration from the outer two-thirds of the disc. The embryonic shield is produced by a convergence of internal cells that starts early in the fifth day at 10" C, augmented by a massive descent of consolidated internal cells from the central one-third of the blastodisc which then form the forebrain and optic vesicles. The cellular envelope is not involved in an invagination, nor does i t take part in the general convergence movements. Chalk grains incorporated into the lower surface of the germ ring and the embryonic shield reveal the destination of the cells which carry them along. The hypoblast of the ring continues to converge toward the shield until the closure of the yolk plug on the ninth day. The epiblast of the ring meanwhile forms (the cell layers of the yolk sac. Head mesoderm, pharynx endoderm and notochord materials are assembled directly into the embryonic shield while the germ ring is forming. Additional evidence is presented thait these parts are not formed by invagination. Materials of the hypoblast originally placed as far as 90" out on the ring from the early shield reach the trunk endoderm, and materials from the hypoblast of the entire ring are passed along toward and into the somite and lateral plate strips. Their oblique paths of migration, combining both epiboly and convergence, are at marked angles to the simple epibolic paths of the epiblast cells, and it is suggested that the split between epiblast and hypoblast in the germ ring is a result of this shearing effect. This paper pursues questions which arose out of the observations of Ballard and Dodes ('68) about the origin and fate of the hypoblast components during the morphogenetic movements of the salmonid embryo. In Salmo gairdneri the hypoblast separates from the more superficial cells at the perimeter of the expanding blastodisc during the sixth day of development at 10" C. It can be identified first near the embryonic shield, and the cleft that defines it quickly spreads around the germ ring. Experimental evidence has been submitted (Ballard, '66a, b,c) for the view that this hypoblast is not invaginated from the surface, and that it is not formed by any similar movement of wheeling-in, such as one might imagine being executed by internaI cells that lie just under the cellular envelope at the perimeter. Instead, the evidence so far suggests that more centrally located deep cells move toward the periphery to take part in the nubbin, the embryonic shield and the hypoblast.The newly described detail and the precise time schedule now av...