THE BEGINNINGS OF THINGS (_continued_)
It is not necessary to describe in detail the beginnings of all the
various structures which arise from that important layer of cells in the
embryo which is termed the ectoderm; but since it gives rise to that
part of the embryo, which eminently places man in the first place in the
world of animals, we may select it for a little further description. We
may leave out of account the beginnings of the skin and the glands, and
some other parts, and look for the moment at the origin of the nervous
system, which includes the brain, the spinal cord, and the whole nervous
mechanism of the individual. Since man's prominence depends upon the
wonderful capacities in his nervous system, it is all the more
interesting to note from what small and simple beginnings it has arisen.
As we have already seen, at a very early stage in the development of the
embryo, a folding of its cells takes place, so that the upper embryonic
area assumes the character of a groove. We may confine our attention to
this groove for the moment, leaving out of account the other two layers
of the embryo--namely, the mesoderm and the entoderm. It is this groove,
which thus early makes its appearance, which is subsequently to play
such a tremendous part in the formation of the most important
structures. It is called the "medullary groove." As growth proceeds and
the cells continue to multiply and increase in numbers, the two edges or
lips of the groove gradually approximate, and ultimately fuse together.
Obviously the effect of this is to transform what was the groove into a
closed cavity or canal, which is therefore now termed the medullary
canal. Arising in this simple manner, this equally simple structure is
destined to become the central canal of the spinal cord, and the
cavities in the brain, known as the ventricles. The walls of this canal,
be it remembered, are composed of cells of the layer of ectoderm, and it
is these cells which, as we saw, appeared very early in the development
of the embryo that are now to proceed to develop into the brain, spinal
cord, and, in fact, the whole central nervous system. At first the cells
appear all similar, but, as development goes on, they begin to
differentiate themselves into different kinds, some forming the actual
nervous cells of the brain and spinal cord, others developing into
protective structures.
The hinder or posterior part of this medullary groove and canal is
narrower than the anterior portion. This posterior narrower part is that
which gives rise to the spinal cord. It very soon changes its character
by the appearance of a number of constrictions at intervals running
along its whole length. It becomes, as it is termed, segmented. A little
later these successive segments are seen to correspond to the pairs of
spinal nerves which arise from the cord. For the first part of embryonic
life the developing spinal cord is of the same length as the canal, but
as time goes on the canal grows longer than the cord. This involves the
nerves coming from the hinder portion growing longer than others. It is
the front or anterior portion of this medullary canal which is concerned
in the development of the brain itself, and here, at an early stage, two
very obvious constrictions appear in the region of what is to be the
brain, and these constrictions divide that brain area into three
distinct parts, or vesicles. Part of the posterior vesicle ultimately
develops into the _cerebellum_, or little brain. Another part forms the
_medulla oblongata_, that important hind brain in which lie so many of
the vital centres of nervous energy. The central cavity formed by these
constrictions is of comparatively less importance, forming ultimately
what is known as the _mid-brain_. The foremost or anterior vesicle,
however, is of the very greatest importance, and its subsequent changes
are more marked than either of the other two. From it is developed the
great mass of the cerebrum itself, together with various outgrowths from
it which have most important functions. Thus two of these outgrowths
appear projecting from the lower part of the sides of the walls, and
ultimately coming to reach the outer ectoderm. These two projections, or
pouches, ultimately form the optic vesicle. Still later in development
the whole of the anterior vesicle is again constricted, thus forming two
distinct parts, the foremost of which, growing rapidly in two halves on
either side of the middle line, ultimately give rise to the two cerebral
hemispheres. These two cerebral hemispheres, therefore, arise, in the
first place, as lateral enlargements of the anterior part of one of the
primitive constrictions of the medullary canal. In their outer layers
cells continue to make their appearance with great rapidity, and thus
is formed the cerebral cortex; and the remarkable thing about this
all-important part of the brain itself is that all the cells of this
cerebral cortex appear to be produced during the life of the embryo;
there being in all probability none added after birth has occurred. That
is to say, the possibilities of the actual physical growth of brain
tissue in any given embryo are fixed from the beginning. Brain tissue,
in other words, is born, not made. It is the manner in which it is
treated afterwards upon which depends whether that given quantity of
brain-cells displays its best potentialities or not.
[Illustration: FIG. 5.--Diagram of brain at an early stage, showing the
origin of the olfactory lobe, the optic vesicle, the cerebellum, the
cerebrum, the medulla, and the spinal cord (after Martin).]
We have seen that the optic structures are concerned with this front
portion of the developing brain. The same is true of the organs which
are concerned with the special sense of smell; for about the fourth week
of the life of a human embryo there appears on the under surface of each
of the cerebral hemispheres, towards the front, a prolongation which
becomes the olfactory lobes.
It is well known that the surface of the brain of an adult human being,
or, indeed, of any of the higher vertebrates, shows upon its surface a
number of convolutions, and it is generally recognised, from a study of
the comparison of different vertebrate brains, that the more convoluted
is the surface of the adult brain the more highly developed is the
animal concerned, from the point of view of brain power. The surface of
the cerebral hemispheres, however, is quite smooth for some months of
embryonic life, and the depressions which give rise to the appearance of
the convolutions do not show themselves until about the fifth month, at
which stage the brain is relatively large.
We referred on a previous page to the origin in evolution of visual
sensation, and it may be of interest here to note a little more fully
the beginnings of the eye itself in the embryo. As has been said, the
very first appearance of these organs takes the form of a pair of
outgrowths, or processes, which are hollow, from the front part of the
anterior vesicle of the brain. These grow until they reach the ectoderm.
A remarkable change then takes place. The portion of the hollow vesicle
which reaches the outermost embryonic layer becomes folded in upon
itself so as to form a cup with a double wall; just as one might form a
cup in a blown-up paper bag by forcibly pressing one portion of it into
the other. This double-walled cup is of special interest, because from
its walls is ultimately developed that very important structure in
connection with sight, namely, the retina. As soon as this is completed
cells begin to grow from it towards the brain in the form of nerve
fibres, and these in time convert what was originally a hollow process
or growth into a solid mass of nerve tissue. This mass is the optic
nerve. Thus is completed the connection between the outer surface of the
eye and the brain itself, which is to receive the sensation. Then the
ectoderm on the surface over the cup begins to thicken, grows into the
cup itself, and ultimately forms a rounded hollow mass which we
afterwards recognise as the _lens_ of the eye. Still later this becomes
separated from the surface by another layer of cells constituting the
_cornea_, and outside that again is still another layer which makes the
_conjunctiva_.
Subsequently the contents of this cup become filled up from other
sources with a soft gelatinous tissue. Then the eyelids in time make
their appearance in the shape of folds of skin growing over the eye,
and remaining in contact until very shortly before birth occurs. And so
we see that from this wonderful layer of ectoderm there comes gradually
into existence not only the brain itself and the spinal cord, with all
the nerves, but also the special sense organs of sight and smell.