established by Remak and Kolliker. Remak, in determining more closely
the part which the different germinal layers play in the formation of
the various tissues and organs, and in applying the theory of
evolution to the cells and the tissues they compose, raised the theory
of germinal layers, at least as far as it regards the vertebrates, to
a high degree of perfection.

Remak showed that three layers are formed out of the two germinal
layers which compose the first simple leaf-shaped structure of the
vertebrate body (or the "germinal disk"), as the lower layer splits
into two plates. These three layers have a very definite relation to
the various tissues. First of all, the cells which form the outer skin
of the body (the epidermis), with its various dependencies (hairs,
nails, etc.)--that is to say, the entire outer envelope of the
body--are developed out of the outer or upper layer; but there are
also developed in a curious way out of the same layer the cells which
form the central nervous system, the brain and the spinal cord. In the
second place, the inner or lower germinal layer gives rise only to the
cells which form the epithelium (the whole inner lining) of the
alimentary canal and all that depends on it (the lungs, liver,
pancreas, etc.), or the tissues that receive and prepare the
nourishment of the body. Finally, the middle layer gives rise to all
the other tissues of the body, the muscles, blood, bones, cartilage,
etc. Remak further proved that this middle layer, which he calls "the
motor-germinative layer," proceeds to subdivide into two secondary
layers. Thus we find once more the four layers which Baer had
indicated. Remak calls the outer secondary leaf of the middle layer
(Baer's "muscular layer") the "skin layer" (it would be better to say,
skin-fibre layer); it forms the outer wall of the body (the true skin,
the muscles, etc.). To the inner secondary leaf (Baer's "vascular