be present in all eggs, and to coincide with the first plane of cleavage (or with any other cleavage plane), we still could not explain the bilateral symmetry of the one-half and one-fourth whole embryos that come from the corresponding isolated blastomeres. If a preëxisting bilateral plane exists in the egg, it must be reëstablished in some way in the isolated blastomere and in pieces of the blastula wall. In the latter case this could scarcely be brought about by a redistribution of the gross contents of the piece, since the presence of cell walls would interfere with such a process. This analysis shows, I think, that the transformation of a piece into a new whole really involves a change in the fundamental structure itself. There cannot be much doubt that both the polarity and the bilaterality of the egg, or of a piece of the egg, belong fundamentally to the same class of phenomena, and we are forced to the supposition that they are inherent peculiarities of the living substance. Driesch thought at one time that it is only necessary to suppose that the protoplasm, and every part of it, possesses a primary polarity, and that some inequality in the material might determine the plane of bilaterality; but later he thought it necessary to assume also the presence of a bilateral structure in the protoplasm, and in all parts of it. This assumption of every part having a polar and a bilateral structure, and the polarity and bilaterality of the whole being the sum total of those of all its parts, is, I think, insufficient to meet the situation. If, for example, the first plane of cleavage coincides with the median plane of the body, the right blastomere has a structure that leads to the formation of the right side of the body, and similarly for the left blastomere. If the two blastomeres are separated, and each gives rise to a whole embryo with a new plane of bilateral symmetry, we must suppose that a new bilaterality has been produced. It does not make the problem any simpler to assume, as Driesch has done, that this is brought about by the elements rearranging themselves bilaterally on each side of a new plane that passes through the middle of the isolated blastomere, for what we need to have explained is what determines the new median plane. It seems to me that the problem is not any simpler, if we assume the polarity and bilaterality to be the property of a large number of elements, as Driesch has done, than if we assume at once the polarity and bilaterality as characteristic of the whole egg. The difficulty of understanding how a new bilaterality can be induced in a piece of the whole is as great on the one assumption as on the other. Not only is it, I think, a much simpler idea to suppose the structure is something pertaining to the whole and is not the sum total of smaller wholes, but the idea is more in accord with other phenomena.