3. Driesch[119] was the first to show that if we isolate one of the first two cells of a dividing egg each develops into a whole embryo of half size. This is perfectly intelligible, since each of the two cells contains all the three layers in the normal arrangement (Fig. 10). The cells divide and the cells having the tendency to creep to the surface of the mass arrange themselves in a hollow sphere, the blastula. Since micromeres and intestine material are present and in their normal position an intestine will grow into the blastula and a whole organism will result. All of this is as necessary as is the formation of one embryo from the whole egg material. Yet the two half-embryos betray their origin from two cleavage cells of the same egg, in that the two gastrulæ formed are often if not always symmetrical to each other (Fig. 15), as the writer had a chance to observe in the egg of _Strongylocentrotus purpuratus_[120] in the following experiment. The eggs of the sea urchin _Strongylocentrotus purpuratus_ are put soon after fertilization into solutions which differ from sea water in two points; namely that they are neutral or very faintly acid (through the CO₂ absorbed from the air) instead of being faintly alkaline, and second, that one of the following three constituents of the sea water is lacking; namely: K, Na, or Ca. When the eggs are allowed to segment in such a solution the first two cleavage cells are as a rule in a large percentage of cases--often as many as ninety per cent.--separated from each other, and when the eggs are put into normal sea water (about twenty minutes after the cell division) each cell develops into a normal embryo. In a number of cases the embryos remained inside the egg membrane and did not move until after the invagination of the intestine was far advanced; in such cases it was found quite often that the invagination began at the plane of cleavage at symmetrical points of the two embryos, and the growth of the intestine was symmetrical in both embryos. [119] Driesch, H., _Ztschr. f. wissnsch. Zoöl._, 1891, liii., 160. [120] Loeb, J., _Arch. f. Entwcklngsmech._, 1909, xxvii., 119. [Illustration: FIG. 15]