not finally settled as regards the animal after the fetal stage, but it is much stronger as regards embryos. In the early stages of many arthropods it is certain that the whole embryo is at first an unmistakable syncytium. This is almost established also for Amphioxus, the Echinoderm Volvox, and other animals. Adam Sedgwick holds that it is true for vertebrates up to a late embryonic stage. Mitosis, then, is a form of growth of a mass, not a generation of new individuals. Whether chromatin or any other element in the germ-cell be the idioplasm in which heredity inheres, differentiation is a progressive transformation, through physical and chemical changes, of the substance of the ovum, and this transformation occurs in a definite order and a definite distribution in the ovum. The changes result in a cleavage of the egg into cells, the boundaries of which sharply mark the areas of differentiation. These cells take on specific characters. In the four-celled stage of an annelid egg these four cells contribute equally to the formation of the alimentary canal and the cephalic nervous system, but only one of them, the left-hand posterior cell, gives rise to the nervous system of the trunk and to the muscles, connective tissues, and germ-cells. The relation between the four original cells, or blastomeres, and the adult parts arising from them, is not fixed, because in some eggs these relations may be artificially changed. A portion of the egg which normally would develop into a fragment of the body will, if split off from the others, give rise to an entire body of a diminished size.