The cell division proceeds and the cells become smaller and smaller and all gather at the surface of the egg, thus forming a hollow sphere. It is not known what brings about this gathering of the cells at the surface, whether it is protoplasmic creeping or streaming or whether the cells are held by a jelly-like layer which covers the surface of the egg (hyaline membrane) (Fig. 13). Then the cilia are formed at the external surface of these cells and the egg begins to swim; we say it has reached the first larval, the so-called blastula stage. This happens according to Driesch after the tenth series of cell divisions, when the number of cells is theoretically 1024, in reality not quite so many (between 800 and 900). The next step consists in the cells derived from the material _A_ (mesenchyme and micromeres) gliding into the hollow sphere, where they form a ring, the physicochemical process responsible for this gliding being yet unknown. At the opening of this ring an active growing of the cells of the entoderm into the hollow sphere takes place and the hollow cylinder formed by this growth is the intestine (Fig. 14). Why the cells grow into the hollow sphere and not into the opposite direction is unknown. The next step is the formation of a skeleton by the formation of crystals consisting of the CaCO₃ by the mesenchyme cells surrounding the intestine. For the establishment of the principle in which we are interested the description of morphogenesis need not be carried farther. [Illustration: FIG. 13] [Illustration: FIG. 14] This principle which is under discussion here is the development of a purposeful arrangement of organs out of the egg. If we assume that the egg consists of homogeneous material we are indeed confronted with a riddle. Since the facts contradict such an assumption but show, as Boveri has pointed out, a prearrangement which allows us to indicate in the unfertilized egg already the exact spot where the intestine will grow into the blastula cavity, we are on solid physicochemical ground, although many questions of detail cannot yet be answered. Such a preformation as Boveri has demonstrated is only conceivable if the material of the egg has not too high a degree of fluidity; we may consider it as consisting essentially of a semi-solid gel which is not homogeneous throughout the egg but divided into three strata.