FERTILISATION AND EARLY DEVELOPMENT
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 assume 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 constitution of these eggs and sperms before this stage is reached, but into these we need not enter. It will suffice for our purpose to assume 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 circumstances 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 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.)
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.)
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.)
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.