of cells, which I call
the _blastula_. The corresponding early ancestor I name the _Blastaea_,
and again we find examples of it, like the _Volvox_ of the ponds, in
Nature to-day.

The next step is very important. The hollow sphere closes in on itself,
as when an india rubber ball is pressed into the form of a cup. We have
then a vase-shaped body with two layers of cells, an inner and an outer,
and an opening. The inner layer we call the entoderm, the outer the
ectoderm; and the "primitive mouth" is known as the blastopore. In the
higher animals a good deal of food-yolk is stored up in the germ, and so
the vase-shaped structure has been flattened and altered. It has,
however, been shown that all embryos pass through this stage
(gastrulation), and we again infer the existence of a common ancestor of
that type--the _Gastraea_. The lowest group of many-celled animals--the
corals, jelly-fishes, and anemones--are essentially of that structure.

The embryo now consists of two layers of cells, the "germ-layers," an
inner and outer. As the higher embryo develops, a third layer of cells
now pushes between the two. We may say, broadly, that from this middle
layer are developed most of the animal organs of the body; from the
internal germ-layer is developed the lining of the alimentary canal and
its dependent glands; from the outer layer are formed the skin and the
nervous system--which developed originally in the skin.

The embryo of man and all the other higher animals now develops a
cavity, a pair of pouches, by the folding of the layer at the primitive
mouth. Sir E. Ray Lankester, and Professor Balfour, and other students,
traced this formation through the whole embryonic world, and we are
therefore again obliged to see in it a reminiscence of an ancestral
form--a primitive worm-like animal, of a type we shall see later. The
next step is the formation of the first trace of what will ultimately be
the backbone. It consists at first of a membraneous tube, formed by the
folding of the inner layer along the axis of the embryo-body. Later this
tube will become cartilage, and in the higher animals the cartilage will
give place to bone.

The other organs of the body now gradually form from the germ-layers,
principally by the folding of the layers into tubes. A light area
appears on the surface of the germ. A streak or groove forms along its
axis, and becomes the nerve-cord running along the back. Cube-shaped
structures make their