The Questions of Developmental Biology

thereby forming the secondary hypoblast (Eyal-Giladi et al.
1992). 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. Thus, although
the shape and formation of the avian blastodisc differ from
those of the amphibian, fish, or echinoderm blastula, the
overall spatial relationships are retained.
The avian embryo comes entirely from the epiblast.
The hypoblast does not contribute any cells to the developing
embryo (Rosenquist 1966, 1972). Rather, the hypoblast cells form
portions of the external membranes, especially the yolk sac and the
stalk that links the yolk mass to the endodermal digestive tube. All
three germ layers of the embryo
proper (plus a considerable amount
of extraembryonic membrane) are
formed from the epiblastic cells.
Fate maps of the chick epiblast are
shown in Figures 11.10 and 1.8
(Schoenwolf 1991).
The primitive streak
The major structural characteristic of avian,
reptilian, and mammalian gastrulation is the
primitive streak. This streak is first visible
as a thickening of the epiblast at the
posterior region of the embryo, just anterior
to Koller's sickle (Figure 11.10A). This
thickening is caused by the ingression of
endodermal precursors from the epiblast
into the blastocoel and by the migration of
cells from the lateral region of the posterior
epiblast toward the center (Figure 11.10B;
Vakaet 1984; Bellairs 1986; Eyal-Giladi et
al. 1992). As these cells enter the
primitive streak, the streak elongates
toward the future head region. At the
same time, the secondary hypoblast
cells continue to migrate anteriorly
from the posterior margin of the
blastoderm. The elongation of the
primitive streak appears to be
coextensive with the anterior migration
of these secondary hypoblast cells. The
streak eventually extends 60 75% of
the length of the area pellucida.