This symmetry is probably due to the following fact: the first cleavage plane goes through that spot where the intestine grows into the blastula cavity. If the micromere material does not change its position after the two cleavage cells are separated and the new blastulæ do not become completely spherical the symmetry which we observed is bound to occur. The occurrence is a confirmation of Boveri’s observation. It is natural that Driesch also found that each cell in the four-cell stage should give rise to a full embryo, since each of these cells is in reality a diminutive egg containing the three strata in the right arrangement. When, however, the cells of the eight- or sixteen-cell stage were isolated Driesch’s results were different. In this case the isolated cells from the ectoderm material did no longer all form a gastrula; when such a cell still formed a gastrula it was probably due to the fact that it contained some entoderm material; while the cells taken from the entoderm region all formed embryos and therefore contained ectoderm material.[121] The isolated ectoderm cells of a blastula could no longer form an intestine; they were lacking the entoderm material. It looks as if a gradual migration of all the entoderm material from the ectoderm into the entoderm took place during the blastula formation. [121] Driesch, H., _Arch. f. Entwcklngsmech._, 1900, x., 361. When the contents of the egg are displaced by pressure the result will be determined by the location of the main mass of the intestine-forming material; where the main mass of this body is located the invagination of the intestine will take place. In his earlier work Driesch assumed from pressure experiments that the egg had a great power of “regulation.” In a later paper[122] he expressed to a large extent his agreement with Boveri who denied this power of “regulation” and showed that the existence of the structure of the egg--_i. e._, a division into three strata, one forming the ectoderm, the second the entoderm, and the third the mesoderm--was sufficient to explain the various phenomena of apparent “regulation.” Driesch’s idea of a regulation in this case has often been used to insist upon the non-explicability of the phenomena of development from a purely physicochemical viewpoint. It is, therefore, only fair to point out that Boveri[123] has furnished the facts for a simpler explanation, which seems to have escaped the notice of antimechanists.[124] [122] Driesch, H., _Arch. f. Entswcklngsmech._, 1902, xiv., 500. [123] Boveri, Th., _Verhandl. d. physik. med. Gesellsch._, Würzburg, N.F., 1901, xxxiv., 145. [124] v. Uexküll makes in his last book (_Bausteine zu einer biologischen Weltanschauung_, München, 1913, p. 24) the following statement: “Driesch succeeded in showing that the germ cell has no trace of a machine-like structure but consists entirely of equivalent parts.” This is not correct.