EARLY DEVELOPMENT
The early development of the embryo now proceeds rapidly, and its appearance at the stage we have just been describing is thus stated by Dr. R. W. Johnstone:—
“If the ovum at this stage be looked at from above, the embryonic area appears as a small shaded oval. The shading is due to an increased growth of cells, because here the three germinal layers—embryonic ectoderm, mesoderm, and entoderm—are in contact. At one end a patch of darker shading indicates a still greater growth of cells. Running forward from this is a band—the primitive streak—in the centre of which lies a darker line—the primitive groove. At the far (anterior) end of the primitive groove there is a dark spot—Hensen's node—from which still another streak runs forward, the head process. Later, in front of the primitive streak, a thickened band of ectoderm appears, broadening out posteriorly. The edges of this band rise up to form two folds, which meet anteriorly. The groove between them is the medullary groove, and ultimately they fold over and unite to form the neural canal. (See Fig. 4.)
“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.)”
Fig. 4.
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.