r to me the
deficiencies in previous expositions of the argument from Embryology, I
can now afford to take only a very general view of the more important
features of this argument as they are successively furnished by all the
later stages of individual development. But this is of little
consequence, seeing that from the point at which we have now arrived
previous expositions of the argument are both good and numerous. The
following then is to be regarded as a mere sketch of the evidences of
phyletic (or ancestral) evolution, which are so abundantly furnished by
all the subsequent phases of ontogenetic (or individual) evolution.

The multicellular body which is formed by the series of segmentations
above described is at first a sphere of cells (Fig. 40). Soon, however,
a watery fluid gathers in the centre, and progressively pushes the cells
towards the circumference, until they there constitute a single layer.
The ovum, therefore, is now in the form of a hollow sphere containing
fluid, confined within a continuous wall of cells (Fig. 41 A). The next
thing that happens is a pitting in of one portion of the sphere (B). The
pit becomes deeper and deeper, until there is a complete invagination of
this part of the sphere--the cells which constitute it being
progressively pushed inwards until they come into contact with those at
the opposite pole of the ovum. Consequently, instead of a hollow sphere
of cells, the ovum now becomes an open sac, the walls of which are
composed of a double layer of cells (C). The ovum is now what has been
called a gastrula; and it is of importance to observe that probably all
the Metazoa pass through this stage. At any rate it has been found to
occur in all the main divisions of the animal kingdom, as a glance at
the accompanying figures will serve to show (Fig. 42)[14]. Moreover many
of the lower kinds of Metazoa never pass beyond it; but are all their
lives nothing else than gastrulae, wherein the orifice becomes the mouth
of the animal, the internal or invaginated layer of cells the stomach,
and the outer layer the skin. So that if we take a child's india-rubber
ball, of the hollow kind with a hole in it, and push in one side with
our fingers till internal contact is established all round, by then
holding the indented side downwards we should get a very fair anatomical
model of a gastraea form, such as is presented by the adult condition of
many of the most primitive Metazoa--especially the lowe