single layer of cells, such as the volvox, the magosphaera, synura,
etc. We shall speak further of the great phylogenetic significance of
this fact in Chapter 2.19.

A very important and remarkable process now follows--namely, the
curving or invagination of the blastula (Figure H). The vesicle with a
single layer of cells for wall is converted into a cup with a wall of
two layers of cells (cf. Figures G, H, I). A certain spot at the
surface of the sphere is flattened, and then bent inward. This
depression sinks deeper and deeper, growing at the cost of the
internal cavity. The latter decreases as the hollow deepens. At last
the internal cavity disappears altogether, the inner side of the
blastoderm (that which lines the depression) coming to lie close on
the outer side. At the same time, the cells of the two sections assume
different sizes and shapes; the inner cells are more round and the
outer more oval (Figure I). In this way the embryo takes the form of a
cup or jar-shaped body, with a wall made up of two layers of cells,
the inner cavity of which opens to the outside at one end (the spot
where the depression was originally formed). We call this very
important and interesting embryonic form the "cup-embryo" or
"cup-larva" (gastrula, Figure 1.29, I longitudinal section, K external
view). I have in my Natural History of Creation given the name of
depula to the remarkable intermediate form which appears at the
passage of the blastula into the gastrula. In this intermediate stage
there are two cavities in the embryo--the original cavity (blastocoel)
which is disappearing, and the primitive gut-cavity (progaster) which
is forming.

I regard the gastrula as the most important and significant embryonic
form in the animal world. In all real animals (that is, excluding the