Gastrulation is a crucial developmental event occurring in all Metazoa
. A simple form of gastrulation can be observed in some diploblastic M
etazoa. The egg of these animals gives rise to a blastula, which is a
hollow, single-layer sphere of epithelial cells. The blastula transfor
ms into gastrula by a progressive change in cell shape known as in inv
agination. A particular region of the epithelium forms a depression in
the blastula. The depression later deepens into a secondary cavity wh
ich communicates with the exterior by a single opening, called the bla
stopore. This results in a concentric arrangement of the embryonic cel
ls into two layers surrounding a digestive cavity. The non-invaginatin
g cells form the ectodermal layer, whereas the invaginating cells form
the endodermal layer. Invagination is thought to be the basic process
by which a blastula-like ancestral organism evolved into a two-layer,
gastrula-like animal. Conceivably, this transformation can be ascribe
d to a cytoskeleton-driven mechanism causing the presumptive endoderma
l cells to modify their shape so as to invaginate. This evolutionary s
cheme implies that endoderm played a more important role than ectoderm
in embryogenesis of primitive Metazoa. A similar trend can be discern
ed in early development of present-day animals: the imvaginating regio
n functions as a major morphogenetic territory in the embryo because i
t contains several gene products that directly or indirectly trigger g
astrulation.