Differentiation is now set up. In the blastula stage all the cells are alike, actually and potentially. But soon one part of the hollow ball of cells becomes pushed inwards, and the cells of this inturned layer become different from those of the external layer, while cells of a third kind appear in the space between the external and internal layers. This is the process of differentiation leading to the development of the various tissues--protective, sensory, digestive, skeletal, etc. The cells still continue to divide and grow to their maximal size, but when the process of differentiation begins, the cells which are formed are not quite the same as those from which they originated. Finally, however, when the rudiments of all the tissues of the adult body have been laid down, the cells begin to produce daughter-cells of only one kind. Growth of the embryo consists, therefore, of the dissociation or division of the substance of the ovum and blastomeres, followed by a gradually increasing differentiation of the cells so produced. Reintegration proceeds all the time. Blastula and gastrula larvæ are really organisms capable of leading an independent existence--that is, they are autonomous entities or individuals. The activities of the parts of which they are composed--ectodermal locomotory cells, ectodermal sensory cells, endodermal assimilatory cells, and so on, must be co-ordinated. The cells are in organic material continuity with each other, and events which occur in any one of them affect all the rest. Impressions made upon the sensory cells are transmitted to the locomotory cells, and food-material assimilated by the assimilatory cells is distributed to all the others. At all stages the growing embryo is an organic unity. The more fully it is developed, the greater the morphological complexity of the organism, and the more numerous its activities, the greater is the differentiation; but the greater also is the co-ordination of the organs and tissues. In the higher animals this co-ordination and integration of activities is effected (mainly) by the central and peripheral nervous systems, but specially differentiated nervous cells are not necessary for this purpose. Differentiation during growth is therefore necessarily accompanied by reintegration of the parts dissociated and differentiated.[27]