Embryology Part 2

We may now turn our attention to the consideration of some of the phenomena connected with the early processes in the development of the embryo. We may a.s.sume that the eggs and sperms have reached such a stage in their life history that they are now mature. All that is necessary in order that the development of an embryo should result is that union of the two elements should take place. Many complicated changes have occurred in the const.i.tution of these eggs and sperms before this stage is reached, but into these we need not enter. It will suffice for our purpose to a.s.sume that they are now mature. Then as the result of a natural instinct which suggests certain thoughts and emotions to the male and female animals, which in turn are followed by certain definite acts, the sperm-cell from the male and the egg or ovum-cell from the female are brought into contact. This contact takes place in such circ.u.mstances that the united elements are able to be protected and nourished and so, fertilisation having thus occurred, development begins.

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 const.i.tute 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 s.e.x? 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 s.e.x 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 ma.s.s is termed by embryologists "the morula." (See Fig. 1.)

[Ill.u.s.tration: FIG. 1.]

Next, within this morula some of the cells become condensed into one particular portion, leaving a s.p.a.ce 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.)

[Ill.u.s.tration: 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 ma.s.s 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-ma.s.s. 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.)

[Ill.u.s.tration: 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 s.p.a.ce 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

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 ca.n.a.l_. (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.)"

[Ill.u.s.tration: 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 ma.s.s 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 ca.n.a.l 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 ma.s.s 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 pa.s.s 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 s.p.a.ces 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 superst.i.tions.

The foregoing description of the princ.i.p.al 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

THE BEGINNINGS OF THINGS

We may next turn our attention to the developing embryo at a very early stage, and note from which parts of its growing cells the different structures are ultimately developed, remembering all the while that all the subsequent division into specialised tissues is the result of the inherent possibilities in one single fertilised germ-cell.

It will be remembered that, as the result of the subdivision of the fertilised germ-cell, we had the formation of three distinct layers of cells. These layers we saw were termed the germinal layers, and were named respectively the "ectoderm," the "entoderm," and the "mesoderm"--the last appearing between the two former. It is from these three germinal layers that all the subsequent structures of the body take their origin, and although we cannot attempt to follow out in detail the growth of all these special tissues, it will, nevertheless, be of interest to note, in the briefest possible way, from which portion of the embryo they subsequently arise. Some of these we may afterwards note in detail. The total result may be summarised by simply giving a list of the various tissues, and the corresponding embryonic layer from which they come. Thus:--

A. From the ectoderm arise the following structures:--

Epidermis or skin.

The hair.

Various glands.

The lens of the eye.

The whole nervous system.

The nerve parts of the sense organs.

The membrane of the mouth.

The enamel part of teeth.

The membrane of the nose.

The lower part of the bowel.

B. From the mesoderm arise the following structures:--

The connective tissues of the body.

The bones.