and ontogeny, of comparative psychology and psychiatry. But still there might nevertheless have existed some doubts as to whether some part, at least, of our mental life might not be independent of the brain, and traceable to the activity of an immaterial soul. However, since we have been able accurately to follow the process of fertilization, since we know that even the finest qualities of mind in both parents are transmitted to the child by the act of fertilization, and that this inheritance is determined simply by the commingling of the two copulating cell-kernels, all the doubts referred to above have vanished. It must appear utterly senseless now to speak of the immortality of the human person, when we know that this person, with all its individual qualities of body and mind, has arisen from the act of fertilization, hence that it has a final beginning to its existence. How can this person possess an eternal life without an end? The human person, like every other many-celled individual, is but a passing phenomenon of organic life. With its death the series of its vital activities ceases entirely, just as it began with the act of fertilization. The variations of form and transformations which the fertilized egg must go through within the uterus before it assumes the form of the mammal are exceedingly remarkable, and proceed from the beginning in man precisely in the same way as in other mammals. At first the fructified egg of the mammal acts as a single-celled organism, which is about to propagate independently and increase itself; for example, an Amoeba (compare Fig. 2, p. 193). In point of fact, the simple egg-cell becomes two cells by the process of cell-division which I have already described (Fig. 6 4). T The same process of cell-division now repeats itself several times in succession. In this way, from two cells (Fig. 6 A) there arise four (Fig. 6 B); from four, eight (Fig. 6 C); from eight, sixteen; from these, thirty-two, etc. FIG. 6. First commencement of the development of a mammal's egg, the so-called "yolk-cleavage" (propagation of the egg-cell by repeated selfdivision). A. The egg, by the formation of the first furrow, falls into two cells. B. These by division fall into four cells. C. These latter have fallen into eight cells. D. By continued division a globular mass of numerous cells has arisen, the Morula. Each time the division of the cell-kernel or nucleus precedes that of the cell-substance, or protoplasma. As the division of the latter always commences with the formation of a superficial annular furrow, or cleft, the whole process is usually called the furrowing of the egg, or yolk-cleavage, and the products of it, that is, the cells arising from the continued halving, are called the cleavage spheres (Blastomera). However, the whole process is nothing more than a simple, oft-repeated division of cells, and the products of it are actual, naked cells. Finally, through the continued division or "furrowing" of the mammal's egg, there arises a mulberryshaped ball (Morula), which is composed of a great number of small spheres, naked cells, containing kernels (Fig. 6 D). These cells are the materials out of which the body of the young mammal is constructed. Every one of us has once been such a simple mulberry-shaped ball, composed only of small cells. The further development of the globular lump of cells, which now represents the young body of the mammal, consists first in its changing into a globular bladder, as fluid accumulates within it. This bladder is called the germbladder (Blastula or Vesicula blastodermica). Its wall is at first composed merely of homogeneous cells. But soon, at one point on the wall, arises a disc-shaped thickening, as the cells here increase rapidly, and this thickening is now the foundation of the actual body of the germ or embyro, while the other parts of the germ-bladder serve only for its nutrition. The thickened disc, or foundation of the embryo, soon assumes an oblong, and then a fiddleshaped form, in consequence of its right and left walls becoming convex (Fig. 7, p. 349). At this stage of development, in the first form of their germ or embryo, not only all mammals, including man, but even all vertebrate animals in general-birds, reptiles, amphibious animals, and fishes -can either not be distinguished from one another at all, or only by very unessential differences, such as size and the arrangement of the egg-coverings. In every one the entire body consists of nothing but two thin layers of simple cells; these lie one on the top of the other, and are therefore called the primary germ-layers. The outer or upper germlayer is the skin-membrane (exoderm), the inner or lower the intestinal membrane (entoderm). The germinal form of the animal body, which thus consists merely of the two primary germ-layers, is the same in all of the many-celled animals (Metazoa), and hence is of the utmost importance. I was the first to maintain the general |