THE BEGINNINGS OF THINGS
We may next turn our attention to the developing embryo at a very early
stage, and note from which parts of its growing cells the different
structures are ultimately developed, remembering all the while that all
the subsequent division into specialised tissues is the result of the
inherent possibilities in one single fertilised germ-cell.
It will be remembered that, as the result of the subdivision of the
fertilised germ-cell, we had the formation of three distinct layers of
cells. These layers we saw were termed the germinal layers, and were
named respectively the "ectoderm," the "entoderm," and the
"mesoderm"--the last appearing between the two former. It is from these
three germinal layers that all the subsequent structures of the body
take their origin, and although we cannot attempt to follow out in
detail the growth of all these special tissues, it will, nevertheless,
be of interest to note, in the briefest possible way, from which portion
of the embryo they subsequently arise. Some of these we may afterwards
note in detail. The total result may be summarised by simply giving a
list of the various tissues, and the corresponding embryonic layer from
which they come. Thus:--
A. From the ectoderm arise the following structures:--
Epidermis or skin.
The hair.
Various glands.
The lens of the eye.
The whole nervous system.
The nerve parts of the sense organs.
The membrane of the mouth.
The enamel part of teeth.
The membrane of the nose.
The lower part of the bowel.
B. From the mesoderm arise the following structures:--
The connective tissues of the body.
The bones.
The teeth, except the enamel.
All the muscles.
All the blood-vessels of the circulation.
All the vessels which carry lymph.
The membranes of the heart, lungs, and bowels.
The kidneys and their ducts.
The reproductive organs.
The blood itself.
The fat and the marrow.
C. From the third layer of the embryo, the entoderm, arises:--
The lining of the alimentary tract.
The lining of the larynx.
The lining of the trachea and lungs.
The cells of the liver, the pancreas, the thyroid, and thymus.
The structure termed the notochord.
From the above very brief summary we see that the body of the
individual, with all its component tissues and parts, can be divided, as
regards its origin, into three groups according as to which embryonic
layer was concerned in its development. Moreover, if these three groups
be scrutinised a little more carefully, they will be seen to differ very
markedly from each other in the structures and tissues which are derived
from them. Thus the structures from the entoderm (see C) are practically
either in the nature of glands, or the lining of the alimentary tract.
Those tissues coming from the mesoderm (see B), on the other hand,
comprise most of what may be termed the supporting tissues of the body,
such as the bones and the muscles and ligaments, as well as the vessels
which constitute the great circulation of the blood and lymph. But
perhaps the most remarkable of all is the list of structures which take
their origin from the ectoderm of the embryo (see A). In this list will
be found the most important structures in the whole human body, as well
as some of those which are apparently of far less serious importance. It
is rather surprising to find, for example, that the whole of the nervous
system, including the brain and spinal cord, and the organs of special
sensation, should be derived from the same layer of cells as gives rise
to the very simple cells of the skin, which serve merely as a protective
covering to the other tissues. It is curious also to observe that in
addition to brain and skin, parts of the teeth also arise from this
external layer. Evidently then this ectoderm or outer layer is of the
very greatest importance in embryology, since from it arise all those
parts of the embryo itself which are the most important in its future
life.