Embryology: The Beginnings of Life

CHAPTER V

The characters of these two wonderful cells, which by their union ultimately cause the production of an embryo, are briefly as follows. The element from the male, the sperm that is, is an extremely minute cell which is only about ¹⁄₃₀₀ of an inch in length. As seen under a high power of the microscope it is composed of two portions which are spoken of as a head and the tail. The former is a flat, oval part, and behind this is the rounded body ending in the long tail which is some four-fifths of the total length. This long tapering tail gives to the sperm its power of movement, for it is supposed that as the result of the rotating or lashing movements of this tail the cell is propelled. Indeed its rate of motion has been actually studied, and estimated to be at about one-eighth of an inch per minute.

The cell contributed by the female, the ovum that is, has quite a different structure and microscopical appearance. Compared with most cells it is rather large, almost round in shape, having a diameter of about ¹⁄₁₂₀ of an inch. Up to the time we are now considering, this cell, along with a great many others like it, has been stored within the female ovary, from which organ an ovum periodically escapes. Unless fertilisation takes place by union with a sperm the discharged ovum perishes. Should, however, the sperm-cell be available, and should it have been able to reach a situation at which fertilisation can take place within, the chain of events which constitute development begins. But before fertilisation can take place the ovum has undergone what is called the process of maturation, in which it divides twice, giving off two small portions of itself in the process. The result of this is that half the number of chromosomes in the ovum are lost. This process of maturation has already taken place in the sperm before it leaves the body of the male.

When these two cells meet, the actual fusion of their material takes place, the head of the sperm penetrating into the substance of the ovum, and the body of the sperm completely fusing with the nucleus of the ovum. This gives rise to what is called the “segmentation nucleus.” It will be observed that we now have a cell in which the full number of chromosomes for that particular species is represented once more. But this full number has now been made up from two different sources, half from the elements contributed from the male, and half from those of the female. It is at this stage that the inherited tendencies, carried in the germ-plasm on the two sides of the ancestry, become mingled, and from thenceforward the division of the fertilised cell into many cell-descendants goes on with extreme rapidity.

Two different lines of germ-plasm have thus been intimately mingled, and the actual significance of this mingling has given rise to one of the most acutely debated points in all the problems of heredity. Put into quite plain language that problem is—What is the function of sex? It is no part of our task here to answer that problem, but it is of interest to point out precisely at what stage it occurs in embryology. The obvious answer, however, may be advanced that the function of sex is to mix the characters of the parents in such a way that some from each source will be found in the offspring. But how these are mixed, whether as painters mix two colours and produce a third, or as two packs of cards are mixed having different coloured backs, is quite another matter.

The fertilised ovum now commences to form a number of successive generations of cells, and this it does by dividing into two, four, eight, sixteen, thirty-two, and so forth, until a number of cells have been produced which arrange themselves into the form of a ball. The surface of this ball resembles that of a mulberry, each elevation corresponding to a cell. This mass is termed by embryologists “the morula.” (See Fig. 1.)

Next, within this morula some of the cells become condensed into one particular portion, leaving a space which contains fluid. The ball is now no longer solid, but has a portion consisting of cells, and a portion consisting of fluid. It is now called a “blastocyst.” (See Fig. 2.)

The cross-section of this shows the cells projecting into a cavity. This is the first attempt of the fertilised ovum to form itself into the different layers, which are ultimately going to give rise to all the different tissues of the embryo. But it is interesting to know at this stage that the outer layer of cells, those representing a margin in the figure, has nothing to do with the forming of the embryo at all, but gives rise to a structure whose function afterwards is to be that of nourishing the growing embryo.

The next obvious change is that the cells at the lower portion of the mass which projects into the cavity appear to get flattened out—at any rate they obviously arrange themselves in a definite and separate layer; and this layer in its turn proceeds to go on growing by division of its cells in such a way as to form another little closed cavity within the larger one. This cavity is termed the “yolk sac.” (See Fig. 3.) Then another little cavity occurs, this time within the original projecting cell-mass. This cavity is termed by embryologists the “amniotic cavity,” and the cells which line it, and which in their turn become arranged as a separate layer, form what is termed the “embryonic ectoderm.” (See Fig. 3.)

It is in this region, and in that of the yolk sac which lies just underneath it, that the future growth of the embryo itself occurs, and the portion is therefore termed the “embryonic area.” (See Fig. 3.)

Up to this point we have seen that two layers of cells have appeared, one round the yolk sac, called the “entoderm,” and the other lining the amnion, called the “ectoderm.” After these two germinal layers have made their appearance, a third layer comes into existence, which, because it begins growing from the embryonic area, and lies between the two already mentioned, has received the name of the “mesoderm.” This third germinal layer divides into two portions before very long, and the space between these two is that in which the body cavity itself subsequently arises. One part of the mesoderm, situated near one end of the embryonic area, is specially important, because in it are formed the blood-vessels which supply the embryo, and which ultimately afterwards becomes the “umbilical cord,” which forms the connection between embryo and mother.

CHAPTER VI

“Along the line of the primitive streak all three germinal layers are in contact. Superficial to it is the amnion, and below it is the yolk sac. The embryonic area is the only part of the ovum which has to do with the subsequent development of the embryo; the other parts of the blastodermic vesicle become subservient as nutritive or supporting structures.

“At this stage, and for the first three weeks of its existence, the embryo is a ‘flat disc floating on the surface of the yolk sac.’ (M'Murrich.)”

This is followed by a folding of the embryo, due to the enlarging of the amniotic cavity, the result being to form what may be termed a “head-fold” and a “tail-fold.” A further fold, however, occurs at the sides which bend in, so that the whole embryonic mass at this stage comes to form an incomplete tube, the incomplete portion being the lower aspect of that tube. This remains open. In due time this lower, or ventral portion, becomes completely closed, except just at one point. This point is where the communication exists between the inside of the tube, which is the embryo, and the yolk sac. A part of the yolk sac is thus included in the embryo itself, and this has an important bearing upon future development, because in the course of time this part comes to be the alimentary tract of the growing embryo. The canal which joins the yolk sac to the internal gut of the embryo (the vitelline duct) ultimately forms, together with part of the yolk sac, the umbilical cord. This cord, which at the time of birth is artificially severed in order to free the fully developed embryo, is at this stage connected to the hinder part of the body of the embryo. As the latter grows, however, it elongates still more behind, in what we should regard as the tail region in animals which had a well-marked tail. As a matter of fact, at a little later stage than this there is quite a conspicuous tail in the human embryo, which, however, comes to be embedded in the tissues later on, and so never forms any external appendage.

So that at this stage we have the embryo representing a mass of cells which have gradually arranged themselves, and been arranged, in the form of a tube more or less bent, and attached near its hinder end to the tissues which are afterwards to represent the umbilical cord.

We have neglected to describe the organs and structures which are developed after fertilisation as a further means of protecting the developing embryo. We have done this of set purpose, because these structures—known as the “trophoblast”—require a considerable amount of technical knowledge to understand. Any detailed description of them, therefore, would be out of place here. All that is necessary for us to say is that they are intended to serve as a means of nutrition for the developing embryo, and take no part in the actual formation of its cells and organs. One portion of it, however, has another function which may be mentioned. It secretes, it is supposed, a kind of ferment which has the power of dissolving or digesting other cells, and this is of great importance at one stage of development—namely, when the fertilised ovum comes to reach the womb, or uterus, in which it is to pass the rest of its developing stage. It is believed that some of the cells in the wall of the uterus are dissolved and digested immediately round the ovum itself, which thus comes to lie in a cavity in the uterine wall. This process being carried still further allows the ovum to sink deeper and deeper into the lining membrane of the uterus. Ultimately the point of entrance, where the cells were digested, is closed up by the formation of a clot of blood poured out at that spot, and which thus entirely covers in the ovum. The latter now comes to lie absolutely embedded in the wall of the uterus in a cavity which it has itself formed. It does not, however, occupy the whole of the cavity, but is surrounded by blood which is escaping from the minute blood-vessels of the wall in which the cavity has been made. This blood is, of course, the maternal blood. “Thus we have the ovum completely embedded, lying free in a tiny cavity in the mucous membrane lining the uterus—a cavity full of blood, in which the ovum lies bathed, and from which it presumably absorbs nourishment by osmosis through its trophoblast.” (R. W. Johnstone.)

The uterine wall, after this embedding of the ovum within it, undergoes a remarkable growth at this position, concerning which a word must be said. Under normal conditions this wall is smooth, or nearly so, but probably there are upon it some slight irregularities or projections which are sufficient to catch the ovum when it enters the uterine cavity. Apparently it may be arrested in this way at any part of the wall, and at that spot it becomes embedded in the manner we have described above. The lining membrane of the uterus under ordinary conditions measures about one-eighth of an inch in thickness, but, after the ovum has become embedded in it, it begins to increase until it reaches as much as half an inch. Underneath this lining membrane lies the muscular part of the uterine wall. The ovum itself is embedded about the middle depth of the lining membrane, but as it continues to grow, and increases in size and dimensions it projects more and more into the uterine cavity, that being the direction of least resistance. Before very long the embryo, as it now is, has reached such a size in its growth that it entirely fills the cavity of the uterus. This stage is reached after the third month of gestation.

Another structure, concerning which just a word must be said, is that known as the “placenta,” or more commonly as the “after-birth.” We need only say that this is first developed by means of a number of little outgrowths by means of which the early embryo is attached to the wall of the cavity in which it lies. These outgrowths grow into the uterine tissue around the ovum, and they allow of blood circulating between them. They have, as a matter of fact, two distinct functions to perform—first, that of fixing the ovum in position, and, secondly, they allow of the maternal blood circulating in the spaces between them, and it is from this blood that the embryo derives its nourishment. The blood-vessels ultimately connect with those of the umbilicus, and thence reach the embryo. This organ, the placenta, at the time the embryo is fully developed at birth, is a round structure about nine inches across, and not quite an inch thick in its middle, becoming thinner towards the edges. The surface of it next to the infant is smooth and shiny, beneath which it is rough, that next to the maternal structures being dark-coloured, somewhat like flesh. When the child is born, the severing of the umbilical cord allows the placenta to remain behind in the uterine cavity, whence it is usually expelled shortly afterwards. Should, however, this not be done, and the embryo and the placenta be born together, the child is said to be “born with a caul,” an event which has given rise to many superstitions.

The foregoing description of the principal events in the development of the embryo will be sufficient for our purpose here. Further details on the subject would necessitate a considerable knowledge of physiology and anatomy, and those readers who desire to study the details of the subject further may do so in any of the various works referred to in the bibliography appended to this book.

CHAPTER VII

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.

CHAPTER VIII

It must be remembered that quite a large number of the characteristics that we usually associate 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 associated 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]

With this passing reference to the fact that some important parts of an individual only grow to their full manifestations after embryonic life, we may pass 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 pronounced 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

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.

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.

CHAPTER X

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 passing 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 partition 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 partition allows of the blood from one side of the heart passing to the other. Another partition 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 partition. 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 partition 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 assumes 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 canals 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 canal by the closing of its edges, the upper part ultimately forming the tympanic cavity, and the rest of it remaining as the Eustachian canal. This canal 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, masses 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

Three sets of structures are concerned in human embryonic nourishment, namely, the Allantois, the Villi of the Chorion, and the Placenta.

The Allantois is developed in the form of a hollow bud from the posterior part of the primitive alimentary canal, and ultimately comes to form the umbilical cord, and the embryonic part of the placenta. It is this structure, the allantois, which allows at a very early date of the embryo establishing a blood-connection with the maternal tissues, and hence it plays a very important part in the transmission of nourishment to the embryo. Not only does it do this, but it allows of the removal of waste products.

The villi of the chorion are outgrowths by means of which the very early embryo attaches itself to the walls of the cavity, which it has made for itself in the wall of the uterus. As they grow larger, these villi push their way into many of the small blood-vessels in the uterine wall, and so come to lie actually in a mass of blood from which they abstract the elements of nutrition. At first the villi themselves contain no blood-vessels. Nourishment passes through them by a simple process of osmosis. Later on, vessels grow into the villi themselves. The nutriment supply is secretion, in the first place, of the uterine glands, which these villi easily absorb. This process takes place during the first two or three months of embryonic life. At the end of this time most of the villi disappear, and the few that remain take part in forming the fœtal or embryonic portion of the placenta.

After the third month the embryo is nourished by the placenta itself, which is at this stage developed. As we have seen, it arises partly from the villi of the chorion, which is its embryonic portion. The other part of it is maternal in origin, arising from the portion of the uterine wall which is immediately over the embryo. The connection between this structure, the placenta and the embryo, is constituted by means of the umbilical cord. The function of the placenta is partly to supply nutrition, partly to serve as an organ of respiration for the embryo, whose lungs are, of course, not functional, and partly it acts in the same way as the kidney does in after life, by excreting certain products. From the placenta the embryo derives those food elements at first provided by the secretion of the uterine glands. Afterwards these elements are supplied by cells which lie between the fœtal villi and the blood of the mother. Its respiratory function consists in allowing oxygen and carbonic acid gas to pass by osmosis between the embryonic and the maternal blood. The process is exactly analogous to that which takes place between the gills of a fish and the water in which the fish lies. Of course, it will be easily understood that there is as yet no great need for a large supply of oxygen, because the embryo is merely growing, and not using its various organs.

It should be clearly understood that under ordinary conditions of embryonic life there is no direct mixture of the blood of the mother and that of the developing embryo. All the processes which contribute to its growth and maintenance, including those of respiration and excretion, take place through the intermediate structures above mentioned. This is an extremely important point, because it means—and evidently that is the object of the arrangement—that there may be much of an injurious character in the blood of the mother which never reaches the embryonic tissues at all. Doubtless the cells which form the organs of nutrition for the embryo have a capacity for selecting the elements required for purposes of nutrition. It is their business to look after this process. How perfectly it is performed can at once be understood when we recollect how very frequently the tissues of the mother herself are in anything but perfect health, and yet the embryo is born healthy. Were it not for this intermediary process, the embryo could hardly help being poisoned or otherwise injured by all the varied unhealthy products and substances which the ignorance of some mothers allows to be present in their blood during this important period. Even with this means of protection, the maternal blood may be so utterly deficient in nutritive qualities, or so actively injurious from saturation with alcohol, or from some equally toxic substance, that the fluids which reach the embryonic cells may be very much impaired in quality. Nevertheless, it is astonishing how much danger can be avoided in this way by Nature's provision in the method of nourishing the embryo.

If the development and growth of the embryo in a human being runs a perfectly normal and uninterrupted course, the following points could be observed at various stages. At the end of the fourth week in growth, the embryo is distinctly curved, so that the two ends—the head and the tail—are close together, the whole being about half an inch in length. Even at this very early stage, the canal which gives rise to the brain and spinal cord is closed in. The vesicles of the eye and the ear have both made their appearance, and the limbs are just beginning to show as buds. The heart is quite obvious, and its division into its four chambers is commencing. In another four weeks the embryo has reached the size of one inch, and the head is beginning to take on a shape more resembling that associated with a human being. The tail, on the other hand, has now disappeared. The limbs have grown to the extent that both hands and feet are starting growth, and in the region of the head both the eyes and the ears, as well as the nose, can be distinguished. Even at this stage, however, the sex of the embryo cannot be made out. A month later, at the end of the twelfth week, a considerable development has taken place. The embryo is now about three and a half inches long. There is a general growth to be observed, and the bones are beginning to ossify. In sixteen weeks, when the embryo measures about five inches in length, the sex is easily distinguishable. The most characteristic thing for the weeks succeeding this is the relatively large size of the head, upon which hair appears at about the twenty-fourth week. In twenty-eight weeks the embryo should weigh about 2½ lb., that is to say at the seventh month of embryonic life. Should the child be born at this time as the result of any of the causes which give rise to premature birth, there is a possibility that it may live, though as a rule it does not. Four weeks later it should weigh 3½ lb., and if born now may frequently live, if carefully attended to. In another four weeks the embryo is nearly eighteen inches long, and weighs about 5½ lb., and the body has a more rounded appearance, because by this time there has been a considerable growth in fat. If born at this stage it ought to be quite possible to save the life. Finally, at the end of forty weeks, the normal full embryonic period of human life, the healthy child should weigh about 7 lb., having smooth, pink skin, and being otherwise perfectly developed.

CHAPTER XII

It is quite obvious that the offspring of any species of animal, if they are to live and survive in the same kind of environment as that in which their parents live, must resemble them somewhat closely. The only way in which Nature can secure such a sufficiently close resemblance of offspring to parents is by insuring that they should develop along similar lines. So it is that we find that the whole of the life history of an individual is more or less a recapitulation, with, of course, variations, of that of the parents and ancestors. Each successive step from the very beginning of the fertilisation of the ovum repeats a stage through which previous generations have passed. If from any accident a step in this recapitulation is omitted, the embryo is to that extent deprived of some feature possessed by a parent or ancestor; and if this be a sufficiently important omission, it is impossible for such an embryo to survive. That is one way in which an embryo may differ from its parents. That is a retrogressive change. On the other hand, such an embryo, in addition to recapitulating the stages through which its parents passed in development, may have something new added, something which appears for the first time. In other words a progressive variation may appear.

Now, since the embryo follows the same developmental track as did the parent, passing through the successive stages of germ-cells, fertilised ovum, embryo, fœtus, infant, child, youth, and adult, it follows that should it exhibit any additional peculiarity, unnoted in the parent, the embryo has obviously varied progressively. That is to say, it has pursued the same line of development together with some new addition. On the other hand, should the offspring at any of these stages in its career be obviously without some of the characters of previous generations, it is as certainly due to the fact that the recapitulation of the history of development has been, in that particular, incomplete.

In all successive cases of multicellular organisms, development by a process of repetition of what happened in the previous generation seems to be the rule; and it would appear that only by this means could a mass of cells which constitute an individual grow into something sufficiently like the parents as to be recognised for their offspring. Given the fact that a human individual starts from a single germ-cell, it could only be by following the same steps in development trodden by the parent that the new individual could attain a similar growth. The object of this similarity is, of course, to provide that the offspring may live and survive in an environment more or less similar to that of the parents. As Dr. Archdall Reid puts it, “the embryo starting from the same point, must follow the same road to reach the same goal. The embryo which did not recapitulate the history of the development of a parent would be a monstrosity.”

While, however, recapitulation in development is always more or less clear, it does not follow that it is perfectly complete, nor perfectly identical with the development of the parent. Indeed, on the other hand, there is always a certain amount of variation, either progressive or retrogressive. Progressive variation means that in addition to the development of all the parental stages, something new has been added. Retrogressive variation means that from the total development experienced by the parent, something has been omitted. We are here speaking of characters of a species, and it must not be thought that we are referring to the characters of the embryo as if they were derived from those of their parents. This was clearly pointed out in the earlier portion of our study. The variations in development, to which we here refer, take their origin in the germ-plasm which tends to repeat in each generation similar types of development. In other words the germ-plasm from which individuals spring is of such a nature that the embryos arising from it show in their development a recapitulation of the evolution of their particular species. In addition they may show variations of either a plus or a minus character. These variations are frequently inherited, and persist throughout succeeding generations. In course of time, if there are many of such variations, they accumulate, and to that extent, of course, alter the life history. That is why in watching the development of a human embryo it is impossible to trace accurately the early ancestral development of the race from it. It passes through the stage of a single cell, then becomes multicellular, and gradually assumes the form of a higher and higher type of organism. “Manifestly the additions and subtractions have been vast. It possesses, for instance, a placenta, an organ by which it is attached to the mother, through which it is nourished, and which at one time is larger than the embryo itself; but which, of course, could not have been present in its prototypes. Nevertheless the life history unfolded by the child is just as real, just as complete, and probably more accurate than any written chronicle that attempts to describe the whole past of a race.” (Reid.)

“There is a history in all men's lives
Figuring the nature of the times deceased.”

Here we must conclude this brief sketch of some of the principal facts in the science of Embryology, in the hope that enough has been said to stimulate the interest of our readers in this subject to such an extent that they may be encouraged to pursue its study still further in one of the many textbooks that are devoted entirely, or partly, to this matter.

We would urge in conclusion that the study is well worth while, even for those to whom it has a non-professional interest. It should be sufficiently obvious to any earnest thinker that the problems which are involved in the study of embryology are precisely those which are of the very greatest importance to humanity at large. With this subject is most intimately bound up that of heredity itself, which has been dealt with in another volume of this series. No true understanding of what can be done, or what should be done, in the direction of improving the lot of generations to come, or of making the most of the generation at present existent, can be obtained by any who are absolutely ignorant of these topics. It is only by their study that we realise that the human embryo, which is to become the human individual, consists, to a very large extent, of characters and features which are unalterably settled for it beforehand, to which nothing can be added, and from which nothing can be taken away. In other words, the possibilities for any individual are those which are pre-existent in the germ-plasm from which he or she originates. These possibilities, however, depend upon the environment in which the embryo, infant, child, and adult is subsequently placed for their full expansion. In many directions the inherited tendencies transmitted by the continuity of germ-plasm are unalterably and strictly defined. In many other directions these inherited tendencies can be so modified, drawn out, or even partially suppressed, by suitable surroundings of a hygienic, educative, and moral nature, that if the process be taken in hand sufficiently early wonderful successes may result. These results are those for which the social reformer and the philanthropist and the serious student of sociology are earnestly striving, but it is only by a study of the sciences of Heredity and Embryology that accurate data can be obtained from which justifiable conclusions may be drawn.

The great fact which embryology teaches is that the past is unfolded stage by stage, with certain omissions and additions, so that in very truth—

“The softest dimple in a baby's smile
Springs from the whole of past eternity,
Taxed all the sum of things to bring it there.”

BIBLIOGRAPHY