The first great modelling then occurs. At one side the single layer of cells, of which the wall of the blastosphere is composed, begins to bend inwards, just as a dimple forms in a hollow india-rubber ball if a pin-prick allow some of the contained air to escape. Further cell-divisions occur, and the invagination becomes deeper, until the invaginating wall nearly touches the wall which has retained its primitive position. The embryo has thus become a hollow cup, the walls of which are double. The cup elongates, and its mouth, originally wide open, becomes more and more narrow, until it forms a small pore opening into an elongated blind sack. The embryo in this stage is known as a gastrula. The central cavity becomes the cavity of the gut; the pore leading into it marks the hind end of the future animal, in the case of vertebrates, and is known as the blastopore. The layer of cells lining the cavity of the sack is known as the hypoblast, and gives rise chiefly to the cells lining the alimentary canal of the future animal. The outer layer of cells is known as the epiblast, and forms the outer layer of the skin, and, along the future dorsal line, gives rise to the nervous system. The muscles and skeleton and the reproductive cells arise from a set of cells known as the mesoblast, that are formed chiefly from the hypoblast, and that push their way in between the hypoblast and epiblast. This general course of development may be traced in all members of the vertebrate group, and, with slight modifications, may be applied to a large number of invertebrates. As the modelling of the general contour of the whole body and of the separate organs proceeds, the protoplasm of the cells gradually assumes the characters of the substance of muscle-cells, liver-cells, nerve-cells, blood-cells, and so forth. The problem of this book will become clearer if it be considered with special reference to what goes on in these early stages. Hertwig says that all the cells of the epiblast, hypoblast, mesoblast, and of the later derivatives of these primary layers, receive identical portions of germplasm by means of doubling nuclear divisions. The different positions, relations to each other and to the whole organism, and to the environment in the widest sense of the term, cause different sides of the capacities of the cells to be developed, but they retain in a latent form all the capacities of the species. Weismann says that the nuclear divisions are differentiating, and that the microcosms of the germplasm, in accordance with their inherited architecture, gradually liberate different kinds of determinants into the different cells, and that, therefore, the essential cause of the specialisation of the organism was contained from the beginning in the germplasm. CONTENTS PAGE PREFACE v