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As first described by Pander, the chicken blastoderm originates as a single layer
of cells called the epiblast. Initially, small clusters of cells delaminate from the epi-
thelium, in a process of polyinvagination, to form islands of cells underneath the
epiblast. These cells spread and proliferate to form a continuous layer of extraem-
bryonic tissue called the hypoblast, or primitive endoderm (Fig. 7.3a) (Eyal-Giladi
1984 ). A small, fluid filled space, called the blastocoel (bc), separates the two lay-
ers. A subgerminal space separates the hypoblast from the yolk, which contains a
layer of syncytial nuclei (YSL) (Nagai et al. 2015 ). Koller’s sickle (KS), appears in
the posterior marginal zone (PMZ) as a thickened crescent of extraembryonic cells
(Izpisua-Belmonte et al. 1993 ; Callebaut and Van Nueten 1994 ).
Gastrulation begins in the posterior of the blastodisc, just anterior to the KS. During
gastrulation, movements bring cells from the lateral edge of the blastodisc to the
midline. These movements were named polonaise movements, after the polish dance
in which “couples walk down opposite sides of the hall in two columns, come together
at the end of the hall, clasp hands and go forward in fours down the middle,” in a
double vortex-like process (p. 391) (Gräper 1929 ). Cells converge at the midline on
top of the KS, intercalate and extend toward the center of the epiblast (Voiculescu
et al. 2007 ). Cells lateral to the KS move medially to replace the cells that leave the
KS. These convergence and extension movements bring lateral cells within the blas-
todisc towards the posterior midline in long arcs (Fig. 7.4a, arrows). As cells accu-
mulate, the midline becomes visible as a thickened epithelium, called the primitive
streak (Fig. 7.4a). Within the streak, presumptive mesoderm and endodermal cells
internalize by ingression. During ingression, individual epithelial cells lose their api-
cal–basal polarity, detach from their neighbors and delaminate from the epithelial
sheet as they transition to a mesenchymal state (Levayer and Lecuit 2008 ). As gas-
trulation proceeds, the primitive streak elongates towards the anterior as more cells
converge at the posterior midline (Fig. 7.4a). The anterior limit of the primitive streak
is marked by a thickened knot of epithelium called Hensen’s node, which is derived
from cells in the epiblast and deep cells of Koller’s sickle (Fig. 7.4a) (Lawson and
Schoenwolf 2001 ). After the streak reaches its full extent, the node retracts, migrat-
ing back to the posterior margin leaving condensing notochord in its path.
7.3.1.2 Reptiles
In most reptilian embryos, the early stages of embryonic development occur in
utero and are difficult to observe. Therefore, the molecular and cellular events lead-
ing to gastrulation have not been characterized in as great detail as in other amni-
otes. Most of our understanding of early reptilian development comes from studies
in certain turtle species, which initiate gastrulation soon after oviposition. At the
blastoderm stage, turtle embryos have the same flat, bilamellar structure as avian
embryos, complete with an area pellucida, area opaca and a marginal zone (Hubert
1970 ). These embryos contain a hypoblast formed by polyinvagination (Pasteels
1957 ). Some blastomeres on the edge of the blastodisc deposit their nuclei in the
yolk through incomplete division, forming a yolk syncytial layer (YSL), similar to
that observed in the chicken (Pasteels 1970 ).
W. Tseng et al.