The Questions of Developmental Biology

Invagination. The infolding
of a region of cells, much like
the indenting of a soft rubber
ball when it is poked.
Involution. The inturning or
inward movement of an
expanding outer layer so that it
spreads over the internal surface
of the remaining external cells.
Ingression. The migration of individual cells from the
surface layer into the interior of the embryo.
Delamination. The splitting of one cellular sheet into
two more or less parallel sheets.
Epiboly. The movement of epithelial sheets (usually
of ectodermal cells) that spread as a unit, rather than
individually, to enclose the deeper layers of the embryo.
As we look at gastrulation in different types of embryos, we should keep in mind the
following questions (Trinkaus 1984):
What is the unit of migration? Is migration dependent on the movements of individual cells,
or are the cells part of a migrating sheet or region?
Is the spreading or folding of a cell sheet due to intrinsic factors within the sheet or to
extrinsic forces stretching or distorting it? It is essential to know the answer to this question if
we are to understand how the various cell movements of gastrulation are integrated. For instance,
do involuting cells pull epibolizing cells down toward them, or are the two movements
independent?
Is there active spreading of the whole tissue, or does the leading edge expand and drag the
rest of a cell sheet passively along?
Are changes in cell shape and motility during gastrulation the consequence of changes in
cell surface properties, such as adhesiveness to the substrate or to other cells?
Contrary to expectations, some regional migrational properties may be totally controlled by
cytoplasmic factors that are independent of cellularization. F. R. Lillie (1902) was able to
parthenogenetically activate eggs of the annelid Chaetopterus and suppress their cleavage. Many
of the events of early development occurred even in the absence of cells. The cytoplasm of the
zygote separated into defined regions, and cilia differentiated in the appropriate parts of the egg.
Moreover, the outermost clear cytoplasm migrated down over the vegetal regions in a manner
specifically reminiscent of the epiboly of animal hemisphere cells during normal development.