Embryology: The Beginnings of Life

by Gerald R. Leighton

CHAPTER IX

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.

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.