d from the other two: an antithesis which
has great significance.

Here, instead of dwelling on these details, it will be better to draw
attention to the most general aspect of the facts. Whatever may be the
course of subsequent changes, the first change is the formation of a
superficial layer or blastoderm; and by whatever series of
transformations the adult structure is reached, it is from the
blastoderm that all the organs forming the adult originate. Why this
marvellous fact?

Meaning is given to it if we go back to the first stage in which
_Protozoa_, having by repeated fissions formed a cluster, then arranged
themselves into a hollow sphere, as do the protophytes forming a
_Volvox_. Originally alike all over its surface, the hollow sphere of
ciliated units thus formed, would, if not quite spherical, assume a
constant attitude when moving through the water; and hence one part of
the spheroid would more frequently than the rest come in contact with
nutritive matters to be taken in. A division of labour resulting from
such a variation being advantageous, and tending therefore to increase
in descendants, would end in a differentiation like that shown in the
gemmules of various low types of _Metazoa_, which, ovate in shape, are
ciliated over one part of the surface only. There would arise a form in
which the cilium-bearing units effected locomotion and aeration; while
on the others, assuming an amoeba-like character, devolved the
function of absorbing food: a primordial specialization variously
indicated by evidence.[61] Just noting that an ancestral origin of this
kind is implied by the fact that in low types of _Metazoa_ a hollow
sphere of cells is the form first assumed by the unfolding embryo, I
draw attention to the point here of chief interest; namely that the
primary differentiation of this hollow sphere is in such case determined
by a difference in the converse of its parts with the medium and its
contents; and that the subsequent invagination arises by a continuance
of this differential converse.

Even neglecting this first stage and commencing with the next, in which
a "gastrula" has been produced by the permanent introversion of one
portion of the surface of the hollow sphere, it will suffice if we
consider what must thereafter have happened. That which continued to be
the outer surface was the part which from time to time touched quiescent
masses and occasionally received the collisions consequent on its