The Questions of Developmental Biology

This region, the germinal crescent, does not form any embryonic structures, but it does
contain the precursors of the germ cells, which later migrate through the blood vessels to the
gonads (see Chapter 19).
The next cells entering the blastocoel through Hensen's node also move anteriorly, but
they do not move as far ventrally as the presumptive foregut endodermal cells. Rather, they
remain between the endoderm and the epiblast to form the head mesenchyme and the
prechordal plate mesoderm (see Psychoyos and Stern 1996). These early-ingressing cells all
move anteriorly, pushing up the anterior midline region of the epiblast to form the head process
(Figure 11.12). Thus, the head of the avian embryo forms anterior (rostral) to Hensen's node. The
next cells migrating through Hensen's node become chordamesoderm (notochord) cells. These
cells extend up to the presumptive midbrain, where they meet the prechordal plate. The hindbrain
and trunk form from the chordamesoderm at the level of Hensen's node and caudal to it.
Meanwhile, cells continue migrating inwardly through the lateral portion of the primitive
streak. As they enter the blastocoel, these cells separate into two layers. The deep layer joins the
hypoblast along its midline and displaces the hypoblast cells to the sides. These deep-moving
cells give rise to all the endodermal organs of the embryo as well as to most of the
extraembryonic membranes (the hypoblast forms the rest). The second migrating layer spreads
between this endoderm and the epiblast, forming a loose layer of cells. These middle layer cells
generate the mesodermal portions of the embryo and extraembryonic membranes. By 22 hours of
incubation, most of the presumptive endodermal cells are in the interior of the embryo, although
presumptive mesodermal cells continue to migrate inward for a longer time.