The two-layered blastoderm (epiblast and hypoblast) is joined together at the margin of the area opaca, and the space between the layers forms a blastocoel. Shortly thereafter, a sheet of cells from the posterior margin of the blastoderm (distinguished from the other regions of the margin by Koller's sickle, a local thickening) migrates anteriorly to join the polyinvagination islands, thereby forming the secondary hypoblast ( Eyal-Giladi et al. At this time, most of the cells of the area pellucida remain at the surface, forming the epiblast, while other area pellucida cells have delaminated and migrated individually into the subgerminal cavity to form the polyinvagination islands ( primary hypoblast), an archipelago of disconnected clusters containing 5–20 cells each ( Figure 11.9B). * Some of the marginal zone cells become very important in determining cell fate during early chick development.īy the time a hen has laid an egg, the blastoderm contains some 20,000 cells. Between the area pellucida and the area opaca is a thin layer of cells called the marginal zone (or marginal belt) ( Eyal-Giladi 1997 Arendt and Nübler-Jung 1999). The peripheral ring of blastoderm cells that have not shed their deep cells constitutes the area opaca. This part of the blastoderm forms most of the actual embryo.
At this stage, the deep cells in the center of the blastoderm are shed and die, leaving behind a one-cell-thick area pellucida. This space is created when the blastoderm cells absorb fluid from the albumin (“egg white”) and secrete it between themselves and the yolk ( New 1956). Between the blastoderm and the yolk is a space called the subgerminal cavity ( Figure 11.9A). These cells become linked together by tight junctions ( Bellairs et al. Thereafter, equatorial and vertical cleavages divide the blastoderm into a tissue five to six cell layers thick. As in the fish embryo, these cleavages do not extend into the yolky cytoplasm, so the early-cleavage cells are continuous with each other and with the yolk at their bases ( Figure 11.8E). The first cleavage furrow appears centrally in the blastodisc, and other cleavages follow to create a single-layered blastoderm ( Figure 11.8).
Cleavage occurs only in the blastodisc, a small disc of cytoplasm 2–3 mm in diameter at the animal pole of the egg cell. Like fish eggs, the yolky eggs of birds undergo discoidal meroblastic cleavage. The egg is telolecithal (like that of the fish), with a small disc of cytoplasm sitting atop a large yolk.
The chick embryo can be surgically manipulated and, since it forms most of its organs in ways very similarly to those of mammals, it has often served as a surrogate for human embryos.įertilization of the chick egg occurs in the oviduct, before the albumen and the shell are secreted upon it. Thus, large numbers of embryos can be obtained at the same stage. Moreover, at any particular temperature, its developmental stage can be accurately predicted. It is accessible all year and is easily raised. Ever since Aristotle first followed its 3-week development, the domestic chicken has been a favorite organism for embryological studies.
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