The Questions of Developmental Biology

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).* Some of the
marginal zone cells become very important in determining cell fate during early chick
development.
Gastrulation of the Avian Embryo
The hypoblast
By the time a hen has laid an egg, the blastoderm contains some 20,000 cells. 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). 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,