Cells in the two layers are streaming in different directions. Cells may not normally mix between the epiblast and hypoblast across Brachet's cleft (although they are able to so mix an experimental situation see below). 1, although one cannot always locate it easily with the dissecting microscope (compare Fig. We point out the location of the cleft in the drawing of the 75%-epiboly stage in Fig. 13), or in sectioned material, between the epiblast and hypoblast. Fraser, unpublished observations), an issue for further study by high-resolution time-lapse analysis of whole cell fields of the sort done by Wood and Timmermans.Īs a consequence of the inward movement, a fissure, Brachet's cleft (Ballard, 1980), becomes visible with Nomarski optics (Fig. This movement, termed ingression, could also happen in zebrafish (J. Trinkaus has obtained evidence (unpublished) that in Fundulus, a somewhat larger embryo, involution may not occur at all rather, cells near but usually not just at the margin move from shallow to deeper positions as individuals, rather than in a rolling cell layer. Note that these terms, epiblast and hypoblast, are also used to describe layers of the avian embryonic blastoderm, but the layers so-named seem to be altogether different in these two kinds of vertebrate embryos.Īs a morphogenetic movement, involution at the blastoderm margin seems particularly well documented for the rosy barb (Wood and Timmermans, 1988), a small teleost that develops in many ways very similarly to the zebrafish. Hence, within the germ ring there are two germ layers, the upper, the epiblast, continues to feed cells into the lower, the hypoblast, throughout gastrulation. Involution produces the germ ring by folding the blastoderm back upon itself. Neither is there a blastopore DEL cells involute at the blastoderm margin, which thus plays the role of a blastopore. Just as there was no blastocoele during the blastula period, there is no archenteron in the gastrula. The advance occurs at a nearly constant rate, over an additional 15% of the yolk cell each hour, and providing a useful staging index during most of gastrulation (Fig. During these events, epiboly temporarily arrests, but after the shield forms, epiboly continues the margin of the blastoderm advances around the yolk cell to cover it completely ( Fig. Convergence movements then, nearly as rapidly, produce a local accumulation of cells at one position along the germ ring, the so-called embryonic shield ( Fig.

As a consequence, within minutes of reaching 50%-epiboly a thickened marginal region termed the germ ring appears, nearly simultaneously all around the blastoderm rim ( Fig. The beginning of involution defines the onset of gastrulation, and, so far as we have been able to tell, this occurs at 50%-epiboly ( Fig. Reprinted only by permission of Wiley-Liss, a subsidiary of John Wiley &Įpiboly continues, and in addition, the morphogenetic cell movements of involution, convergence, and extension occur, producing the primary germ layers and the embryonic axis. The 5th edition is available in print and within the ZFIN Protocol Wiki.ĭevelopmental Dynamics 203:253-310. This material is from the 4th edition of The Zebrafish Book.

Definition blastopore