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
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:
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.