Take a small india-rubber ball--not of the inflated kind, nor of the
solid kind, but of the kind about an inch or so in diameter with a small
hole through which, under pressure, the air escapes. Suppose that
instead of consisting of india-rubber its wall consists of small cells
made polyhedral in form by mutual pressure, and united together. This
will represent the blastoderm. Now with the finger, thrust in one side
of the ball until it touches the other: so making a cup. This action
will stand for the process of invagination. Imagine that by continuance
of it, the hemispherical cup becomes very much deepened and the opening
narrowed, until the cup becomes a sac, of which the introverted wall is
everywhere in contact with the outer wall. This will represent the
two-layered "gastrula"--the simplest ancestral form of the _Metazoa_: a
form which is permanently represented in some of the lowest types; for
it needs but tentacles round the mouth of the sac, to produce a common
hydra. Here the fact which it chiefly concerns us to remark, is that of
these two layers the outer, called in embryological language the
epiblast, continues to carry on direct converse with the forces and
matters in the environment; while the inner, called the hypoblast, comes
in contact with such only of these matters as are put into the
food-cavity which it lines. We have further to note that in the embryos
of _Metazoa_ at all advanced in organization, there arises between these
two layers a third--the mesoblast. The origin of this is seen in types
where the developmental process is not obscured by the presence of a
large food-yolk. While the above-described introversion is taking place,
and before the inner surfaces of the resulting epiblast and hypoblast
have come into contact, cells, or amoeboid units equivalent to them,
are budded off from one or both of these inner surfaces, or some part of
one or other; and these form a layer which eventually lies between the
other two--a layer which, as this mode of formation implies, never has
any converse with the surrounding medium and its contents, or with the
nutritive bodies taken in from it. The striking facts to which this
description is a necessary introduction, may now be stated. From the
outer layer, or epiblast, are developed the permanent epidermis and its
out-growths, the nervous system, and the organs of sense. From the
introverted layer, or hypoblast, are developed the alimentary canal and
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