The Questions of Developmental Biology

The next phase of gastrulation involves the involution of the marginal zone cells while
the animal cells undergo epiboly and converge at the blastopore (Figure 10.7C, D). When the
migrating marginal cells reach the dorsal lip of the blastopore, they turn inward and travel along
the inner surface of the outer animal hemisphere cells. Thus, the cells constituting the lip of the
blastopore are constantly changing. The first cells to compose the dorsal blastopore lip are the
bottle cells that invaginated to form the leading edge of the archenteron. These cells later become
the pharyngeal cells of the foregut. As these first cells pass into the interior of the embryo, the
dorsal blastopore lip becomes composed of cells that involute into the embryo to become the
prechordal plate (the precursor of the head mesoderm). The next cells involuting into the
embryo through the dorsal blastopore lip are called the chordamesoderm cells. These cells will
form the notochord, a transient mesodermal "backbone" that plays an important role in
distinguishing and patterning the nervous system.
As the new cells enter the embryo, the blastocoel is displaced to the side opposite the
dorsal lip of the blastopore. Meanwhile, the blastopore lip expands laterally and ventrally as the
processes of bottle cell formation and involution continue about the blastopore. The widening
blastopore "crescent" develops lateral lips and finally a ventral lip over which additional
mesodermal and endodermal precursor cells pass. With the formation of the ventral lip, the
blastopore has formed a ring around the
large endodermal cells that remain exposed
on the vegetal surface.