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 THE BEGINNINGS OF THINGS (_continued_) We have now considered, as far as is compatible with the character of a work of this kind, the beginning and development of the embryo taken as a whole, and for the remaining part of our study of this subject we may devote our attention to the beginnings of some of the more important organs and functions in the new individual. It will be impossible to deal in detail with all the important parts which ultimately constitute the new personality, but a selection may be made which will give some general idea of how great results spring from very small beginnings. What will be said here it may be hoped will be just sufficient to stimulate the interest of those to whom the subject appeals, and who may then turn their attention to some of the larger works which go into greater detail in this subject, a list of which will be found in the bibliography at the end of this volume.