From the introduction: The purpose of the following work from its inception has been to make as careful a study as possible of the cleavage of the ovum, the formation of the germinal layers and definitive organs, and the axial relations of the ovum to the larval and adult axes. At the time when this work was begun, several years ago, scarcely any attempts had been made to trace the history of individual blastomeres through the entire development to the formation of definitive organs. The early stages of cleavage had received a great deal of attention, but the later stages had been largely neglected; and although the origin and homology of the germ layers was perhaps the most frequently discussed subject in embryology, yet the relation of these layers to the individual blastomeres of the cleaving ovum had been determined in comparatively few cases. Since that time a number of very valuable papers have appeared on this subject of "cell lineage," as Wilson ('92) has aptly termed it. The results of such work are no longer as novel as they were four or five years ago, and yet the general interest in the subject has greatly increased, and that, too, in spite of the fact that there is a growing school of biologists who believe that individual blastomeres have no necessary relation to future organs. The subject of germ layers is no longer so important as it was once considered; in fact, the theory of the homology of the germinal layers has met with so many difficulties of late that it is now generally maintained only in a greatly modified form. However, the fundamental idea which was prominent in germ-layer discussions is of vital interest to-day. In the whole history of the germ-layer theories I see an attempt to trace homologies back to their earliest beginnings. This problem is as important to-day as it ever was, and whether one find these earliest homologies in layers or regions or blastomeres or the unsegmented ovum itself, the quest is essentially the same.