Roux interprets Hertwig’s results as due to the sudden partial post-generation of a part of the injured half of the egg. He thinks that a half-embryo had first developed, and then to this there has been quickly added a part of the missing side. This reply fails, however, to meet Hertwig’s description of the method of development of the embryos. Later work, however, has put us in a position to give a more satisfactory account of the differences between the results of Roux and Hertwig. It seemed to me that the two kinds of embryos might be due to the different positions of the eggs after the operation. It had been shown by Schultze (’94) that if a normal egg in the two-celled stage is turned upside down and held in that position two embryos develop from the egg (Fig. 63, _B_, _C_, _D_). These embryos are united in various ways, and arise presumably one from each of the first two blastomeres. These results have been confirmed by Wetzel, who examined more fully into the early development of the twin embryos. He showed with much probability that the protoplasm rotates in each blastomere, so that in many cases the lighter part flows, or starts to flow, toward the upper hemisphere of the egg. In this way similar protoplasmic regions of the two blastomeres may become separated, and under these circumstances each blastomere gives rise to a whole embryo. A cross-section through one of the segmentation stages of one of these eggs is shown in Fig. 63, _A_. The smallest cells are found at the outer side of each half, and the two segmentation cavities lie one in the upper region of each hemisphere. Some of the different kinds of embryos that develop from inverted eggs are shown in Fig. 63, _B_, _C_, _D_. They are united in Fig. 63, _B_, by their ventral surfaces, and in Fig. 63, _C_, _C¹_, _C²_, by their dorsal surfaces, and in Fig. 63, _D_, _D¹_, at the sides. These differences are probably accounted for by the different ways in which the protoplasm of the first two blastomeres rotated before the egg divided.