brane is
exposed to the air its cell-structure changes into the cell-structure of
skin? The position taken must be this:--Though mucous membrane in a
highly-evolved individual organism, thus shows the powerful effect of
the medium on its surface; yet we must not suppose that the medium had
the effect of producing such a cell-structure on the surfaces of
primitive forms, undifferentiated though they were; or, if we suppose
that such an effect was produced on them, we must not suppose that it
was inheritable. Contrariwise, we must suppose that such effect of the
medium either was not wrought at all, or that it was evanescent: though
repeated through millions upon millions of generations it left no
traces. And we must conclude that this skin-structure arose only in
consequence of spontaneous variations not physically initiated (though
like those physically initiated) which natural selection laid hold of
and increased. Does any one think this a tenable position?

* * * * *

And now we approach the last and chief series of morphological phenomena
which must be ascribed to the direct action of environing matters and
forces. These are presented to us when we study the early stages in the
development of the embryos of the _Metazoa_ in general.

We will set out with the fact already noted in passing, that after
repeated spontaneous fissions have changed the original fertilized
germ-cell into that cluster of cells which forms a gemmule or a
primitive ovum, the first contrast which arises is between the
peripheral parts and the central parts. Where, as with lower creatures
which do not lay up large stores of nutriment with the germs of their
offspring, the inner mass is inconsiderable, the outer layer of cells,
which are presently made quite small by repeated subdivisions, forms a
membrane extending over the whole surface--the blastoderm. The next
stage of development, which ends in this covering layer becoming double,
is reached in two ways--by invagination and by delamination; but which
is the original way and which the abridged way, is not quite certain. Of
invagination, multitudinously exemplified in the lowest types, Mr.
Balfour says:--"On purely _a priori_ grounds there is in my opinion more
to be said for invagination than for any other view";[57] and, for
present purposes, it will suffice if we limit ourselves to this: making
its nature clear to the general reader by a simple illustration.