In connection with these experiments Driesch discussed the factors that determine the axial relations of the embryo. If all the cells have the power of producing all parts, what determines in the normal development, and also in the development of a part of the whole, the axial relations of the embryo? Driesch assumed that the egg has a polar structure, and that the same polarity is found in all parts of the protoplasm. Around this primary axis all the parts are alike or isotropous.[116] The origin of the mesenchyme and the position of the archenteron, that develop at one pole, are determined by the polarity of the protoplasm. The plane of bilateral symmetry may appear in any one of all the possible radial planes around the primary axis. The selection of a particular one is due to some accidental difference in the structure of the protoplasm, or to some external factor. In later papers Driesch modified this view, and assumed that along with the primary polarity a bilateral structure also exists in the protoplasm. Wilson (’93) studied the development of isolated blastomeres of amphioxus, and found that it agreed in all essential respects with the mode of development of the blastomeres of the sea-urchin. The isolated blastomeres of the two-cell and four-cell stages produce whole embryos, but the blastomeres of the eight-cell stage develop only as far as the blastula. The blastomeres segment, after separation, in most cases not as a part, but as a whole egg would divide, although the cleavage of the one-eighth blastomere only approaches that of the entire egg, but is never identical with it. Incompletely separated blastomeres give rise to twins, triplets, etc. Wilson agreed with the Hertwig-Driesch conception of the value of the early blastomeres, and accepted the view that the fate of each is a function of its position, and that at first they are qualitatively alike. During the early cleavage he supposed that a change takes place that is slight at the two-cell stage, greater at the four-cell stage, and in the eight-cell stage the differentiation has gone so far that the blastomere can no longer return to the condition of the ovum. “The ontogeny assumes more and more the character of a mosaic work as it goes forward.” Loeb (’94) showed that if the eggs of the sea-urchin are placed in sea water, diluted by distilled water, the egg swells and bursts its membrane, so that a part of its protoplasm protrudes. Into this protrusion some of the first-formed nuclei pass, and from both the part remaining in the egg membrane, as well as from the protruding part, an embryo is produced, the two embryos often sticking together. In several cases two to eight separate groups of blastomeres are formed from one egg and develop into whole embryos.[117]