A consideration of these results led me to carry out the following experiment on eggs operated upon by Roux’s method. After sticking one of the first two blastomeres, some of the eggs were placed so that the uninjured blastomere kept its normal position, _i.e._ with the black hemisphere upward. Other eggs were turned, so that more or less of the white hemisphere was upward. From the two kinds of eggs two kinds of embryos were obtained. From those with the black hemisphere upward the embryo was a half-embryo like that described by Roux, while from the eggs with the white hemisphere upward embryos developed that were in many respects whole embryos of half size.[112] The explanation of this difference will be obvious from what has been said. When the _black_ hemisphere is uppermost the contents of the uninjured blastomere remain as in the normal egg, and a half-embryo results. When the _white_ hemisphere is uppermost the contents of the uninjured blastomere rotate, so that it generally shifts its relation to the protoplasm in the other injured half, and a whole embryo develops, as in Schultze’s experiment. In one case I obtained a half-embryo from an inverted egg. The result did not appear to be due to a lack of rotation of the protoplasm, because the medullary folds were white, showing that the protoplasm must have changed its position. The result can possibly be explained as due to the protoplasm rotating in each blastomere along the line between the halves, so that it still retains the same relation as that of the normal two-celled stage. The whole embryos of half size are generally imperfect in certain respects on account of their union with the other half. They resemble in all important points the embryos described by Hertwig, and I see no grounds for interpreting them as embryos of a meroblastic type, but rather as whole embryos of half size, whose development posteriorly and ventrally has been delayed or interfered with by the presence of the other blastomere. It has not been possible to separate the first two blastomeres of the frog’s egg, for if one is removed the other collapses. In the salamander, that has a mode of development similar to that of the frog,[113] it has been possible to separate the first two blastomeres. Herlitzka, who carried out this experiment, found that each blastomere gives rise to a perfect, whole embryo of half size. We cannot doubt, I think, that the same power of producing a whole embryo is also present in each of the first two blastomeres of the frog’s egg. When the two remain in contact in their normal relation to each other, each produces only a half; when like regions of the two blastomeres are separated, each produces a whole embryo. Thus we see that whatever the factors may be that determine the development of a single embryo from the egg, still each half, and perhaps each fourth also, has the power of producing a whole embryo.