ays with reference to the
separation of the blastoderm of the chick into two layers. "Yet
originally there are not two distinct or even separable layers, it is
rather the two surfaces of the germ which show this differentiation,
just as polyps show the same contrast of an external surface and an
internal digestive surface. In between the two layers there is in our
germ as in the polyp an indifferent mass" (p. 67). The terms ectoderm
and entoderm were introduced by Allman[330] in 1853 for the two
cell-layers in the Hydrozoa.

Remak is the second great name in the history of the germ-layer theory.
He had the great advantage over von Baer of being able to make use of
the cell-theory in interpreting the formation of the germ-layers.
Microscopical technique also had been greatly improved since 1828.[331]

Remak's greatest service was that he put the germ-layer theory in direct
relation with the cell-theory by demonstrating the cellular continuity
from egg-cell to tissue, and by showing that each germ-layer possessed
distinctive histological characteristics. Hardly less important was his
clear marking-off of the "middle layer" as a separate and distinct layer
of the germ. He it was who introduced the modern conception of the
mesoderm, and cleared up the confusion in which Pander and von Baer had
left the organs formed between the serous and the mucous layer. Remak's
middle layer was a different thing from Pander's ill-defined
"vessel-layer"; it included and unified from a new point of view the
"vessel" and "muscle" layers of von Baer.

There are in the unincubated blastoderm of the chick, according to
Remak,[332] two cell-layers, of which the undermost subsequently splits
into two. Three layers are thus formed--the upper, middle and lower. The
upper layer differentiates into a medullary plate and an epidermic plate
(Remak's _Hornblatt_), and gives origin to the medullary tube with all
its evaginations, and to the skin with all its derivatives and pockets.
It forms such diverse structures as the brain, the spinal cord, the eye,
the ear, the mouth, hairs, feathers, nails, sweat-glands, lacrymal
glands, and so forth. All these parts are connected directly or
indirectly with sensation, and the upper germ-layer may accordingly be
called the _sensory_ layer. The lower layer gives rise to the epithelium
and the proper tissue of the alimentary canal and its derivatives, as
the liver, lungs, pancreas, kidneys, thyroid, thymus, etc.