Embryology Part 3

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 const.i.tute 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.

CHAPTER VIII

THE BEGINNINGS OF THINGS (_continued_)

We have now considered, as far as is compatible with the character of a work of this kind, the beginning and development of the embryo taken as a whole, and for the remaining part of our study of this subject we may devote our attention to the beginnings of some of the more important organs and functions in the new individual. It will be impossible to deal in detail with all the important parts which ultimately const.i.tute the new personality, but a selection may be made which will give some general idea of how great results spring from very small beginnings.

What will be said here it may be hoped will be just sufficient to stimulate the interest of those to whom the subject appeals, and who may then turn their attention to some of the larger works which go into greater detail in this subject, a list of which will be found in the bibliography at the end of this volume.

It must be remembered that quite a large number of the characteristics that we usually a.s.sociate with a normal human being only come into existence, or at any rate only become obvious, at some period longer or shorter after birth. True, these characteristics depend for their ultimate appearance upon the development of the corresponding structures and organs in the growing embryo, but in the case of some of these, those organs are not fully developed in embryonic life, and the manifestation of the functions a.s.sociated with them may be delayed perhaps for years. This is notably the case, for example, with the reproductive organs which, though developed during the life of the embryo, remain functionless until the period of adolescence. The development of the human mind and intellect too, although depending, of course, upon the embryonic growth of the brain and the nervous system generally, is a matter of time and the environment subsequent to birth.

It should be realised, however, in this connection, that the mind of the new individual, and all that is involved in that term, dates back ultimately, as regards its possibilities, to the moment at which the two germ-cells from the male and female respectively united in fertilisation. The adult mind develops from the mind of the infant. The infant mind appears as the result of the possibilities and the tendencies which were inherent in the germ-cells from which not merely the brain but the whole embryo sprang; in other words, all that a single human mind connotes results from the possibilities in a single cell.

Such a thought is a startling one indeed, and at first sight appears, perhaps, somewhat incredible. But a moment's careful attention to the problem will show at once that it is in reality no more wonderful than the fact that this single cell produces all the millions of other cells which in due time give rise to the skin, bone, nerve, blood, and so forth, which make up the entire body of the embryo. The human mind, therefore--and indeed the human soul, if that term be used in any intelligible sense--takes its origin in the products of the multiplications of germ-cells acted upon by their subsequent surroundings.[1]

[1] The detailed study of this part of the subject is dealt with in the writer's work, _The Greatest Life_ (Duckworth, London), to which readers who are interested in this phase of the subject are referred.

With this pa.s.sing reference to the fact that some important parts of an individual only grow to their full manifestations after embryonic life, we may pa.s.s to the consideration of the development of some of the more interesting parts of the embryo itself.

Amongst the most striking, and certainly the most interesting, of the various parts of the developing embryo, those which go to form the special senses are prominent. They are interesting not merely from their actual mode of growth, but especially also in connection with their evolutionary history. The study of how they have come into their present state in the higher animals leads us back to very small beginnings--indeed, to the time when there was no such thing as special sense organs for sight, hearing, smell, and so forth, but where the organism had what may be termed a diffused tactual sense over and throughout the entire body. In the course of time this diffused general sense became specialised, no better example of which could be quoted than that of the sense of sight, which was referred to, as many of our readers will doubtless remember, in Tyndall's famous Belfast address. He was referring to Herbert Spencer's theory of the manner in which vision was evolved. He pointed out that, as above noted, in the lowest organisms sensation is a general thing diffused throughout the body, a kind of general tactual sense. As the result of environment, and gradual adaptation to external influences, certain parts of the general surface of the organism became more responsive to these external stimuli than other parts. These areas, being those points at which sensation was most acutely felt, were nothing more or less than primitive sense organs.

Thus in the progress of evolution the stimulus of the eye gradually became most p.r.o.nounced in certain cells which contain pigment, these cells being more responsive to the light stimulus than the rest of the body. That was the beginning of an eye; a group of cells more receptive, more easily influenced by light, than any other cells. In a slightly higher stage of evolution we find a special overgrowth of the skin which covers over the area in which these pigmented cells lie, obviously a protective measure on behalf of the specialised cells referred to. Then, still later, a lens is added, and the whole organ becomes more and more adapted to the necessities of the case, until it reaches the extraordinary perfection that is seen in the eye of such a bird as an eagle. On the same general principle, the other special senses also took their origin from this general diffused tactual sense, certain cells becoming specially adapted for receiving the stimulus of sound, others for taste, others for smell, and so forth.

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 a.s.sumes 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 ca.n.a.l, which is therefore now termed the medullary ca.n.a.l. Arising in this simple manner, this equally simple structure is destined to become the central ca.n.a.l of the spinal cord, and the cavities in the brain, known as the ventricles. The walls of this ca.n.a.l, 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 ca.n.a.l 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 ca.n.a.l, but as time goes on the ca.n.a.l 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 ca.n.a.l 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 ma.s.s 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 ca.n.a.l. 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 quant.i.ty of brain-cells displays its best potentialities or not.

[Ill.u.s.tration: 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 ma.s.s of nerve tissue. This ma.s.s 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 ma.s.s which we afterwards recognise as the _lens_ of the eye. Still later this becomes separated from the surface by another layer of cells const.i.tuting 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.

CHAPTER X

THE BEGINNINGS OF THINGS (_continued_)

Without entering into the description of the development of the whole circulatory system, we may just mention briefly the origin of the heart itself, which begins at a very early stage by the appearance of a small body of cells, which come to arrange themselves in a tubular form enclosed in the mesoderm. The two halves of this tube are at first quite separate from each other, but gradually come together and finally unite into a single tube with walls. The folds of these ultimately form the heart muscle. The organ, at this stage of its development, does not lie within the region of the chest cavity, as it afterwards does, but more anteriorly in the region of the neck. The simple tubular arrangement, however, is quite a pa.s.sing phase, and as the tube increases in length it becomes bent upon itself, somewhat in the form of the letter S. One end of it now enlarges and forms a pouch on each side, these forming the two auricles, right and left. From these auricles a part.i.tion grows vertically, and when complete, cuts them off from each other, except that a communication is left in the upper part (_the foramen ovale_) which closes up after birth. This part.i.tion allows of the blood from one side of the heart pa.s.sing to the other. Another part.i.tion eventually divides off that portion which has formed the auricles from the remaining portion which develops into the ventricles, which in their turn become again divided by a still further part.i.tion. In this way the heart which, in the first place, was a simple tube, grows ultimately into an organ with four distinct chambers, two auricles and two ventricles, the only difference from this and the adult heart being the communication which exists through the part.i.tion separating the two auricles.

The development of the organ of hearing is somewhat complicated. The first part to appear is a portion of the inner ear, which shows itself as a round, hollowing of the ectoderm. This depression becomes deeper and sinks further in, while its floor becomes thicker, and finally the whole a.s.sumes the shape of a closed cavity. An outgrowth from this gives rise to the _cochlea_. The cavity becomes divided into two portions, in one of which the _semicircular ca.n.a.ls_ arise. Around the whole, the embryonic tissue has been forming into a strong protective covering, some of which finally becomes cartilage, and some bone. The middle portion of the ear is the remains of a cleft in the side of the embryo.

This cleft becomes changed into a ca.n.a.l by the closing of its edges, the upper part ultimately forming the _tympanic cavity_, and the rest of it remaining as the _Eustachian ca.n.a.l_. This ca.n.a.l opens into the pharynx.

In the cavity there are subsequently developed three small bones which play an important part in the process of hearing. After the birth of the embryo, air reaches the tympanic cavity, which then enlarges. One of the walls of this cavity persists as the tympanic membrane or drum. Finally the outer ear, that portion which is popularly spoken of as _the_ ear, is formed from the upper portion of the same cleft which gave rise to the tube of the tympanum.

We have referred in the preceding description to the origin of some embryonic structures from a cleft in the early embryo itself. As a matter of fact, no less than four of these clefts, or fissures, appear in the region of the neck on each side, and are of the very greatest interest and importance in connection with embryology. They are termed the "branchial clefts," and are seen in the embryos of all vertebrates.

In the human embryo there are four. They are situated on each side of the pharynx, and they correspond to the gill slits in lower vertebrates.

Amongst other structures which arise from the important layer of ectoderm are the teeth. Of these there are during life two sets, a temporary and a permanent. The temporary teeth, though they do not make their appearance till after the birth of the embryo, still are partly developed during embryonic life, lying embedded in the tissues until the familiar process known as "cutting the teeth" takes place. This is, of course, merely the time of their external appearance. The first stage in the development, however, is a thickening of the epithelium of the gums in a direction which is to correspond with the line where the teeth will eventually pierce. This thickening is called the "dental ridge." This grows downwards into the underlying tissue in flask-shaped growths. From the neck of each of these flasks there is a small projection which indicates where the permanent teeth will ultimately be. This first stage is termed that of the _enamel germ_. This becomes surrounded by cells which ultimately form a _dental sac_. Next, tissue from below grows into the flask, and the further growth of this gives rise to the _enamel organ_. Finally, enamel itself and dentine are developed, and the embryonic tooth remains covered under the gums until it cuts them.

So far we have considered merely the mode of development of the most important organs of the body, but we have said nothing of the most important supporting structure, namely, the skeleton. The earliest appearance of anything in the shape of a skeleton is the structure known as the notochord, a structure of immense importance and interest in the embryology of all vertebrate animals, in which it is a temporary thing only. The first appearance of this notochord in lower animals is the earliest indication of the vertebrate type, because it is found that in the higher vertebrates it is the forerunner of the bony spinal column and the skull. It appears first as a groove underneath the medullary groove, of which we have already spoken, and its two lips unite to form a cavity, as did those of the medullary groove. In this case, however, the groove becomes a solid rod, then termed the notochord, and it lies immediately under the medullary groove itself, which, as we have seen, gives rise to the central nervous system. In the course of development, ma.s.ses of cells come to arrange themselves on each side of the notochord, which they eventually include, and at the same time they grow upwards and around the spinal cord which is thus enclosed. Later on these surrounding portions become cartilage, and, still later, bone; the notochord meanwhile gradually disappearing where the bony spinal column appears. This primitive vertebrate structure therefore, of the notochord, has the all-important function of coming to enclose, and thus protect, the spinal cord and nervous system.

As regards the other bones of the body, all that need be said here is that they are preceded by the structure which we know as cartilage, and in the bones of the limbs at two or three different points this cartilage begins to be transformed into bone. These points are known as centres of ossification.

CHAPTER XI

HOW THE EMBRYO IS NOURISHED