In later papers Roux has stated that he had also, even in his earlier experiments, found other kinds of embryos than the half-embryos that he described. Some of these were whole embryos that had developed from the uninjured blastomere without the injured one taking any part or only a very small share in their formation. He found, he states, all stages between those embryos that had used up all the yolk material of the injured side (though post-generated) and those that had not used any part of it. The latter kind of embryo he does not recognize as a whole embryo of half size in the sense that a single blastomere has developed directly into a smaller whole embryo, but he believes that there must have been formed at first a half-blastula, half-gastrula, half-embryo, and that the last stage completed itself laterally without using any material from the injured half. That the uninjured blastomere may at first segment as a half is not improbable, but that whole embryos are formed only by the formation of new material at the side of a half-embryo is, I think, hardly possible, since the results of Schultze, Wetzel, Hertwig, and myself show that a whole embryo may develop _directly_ out of the material of a single blastomere. Spemann (1900) has carried out some novel experiments on the eggs of triton, and has shown how in another way double structures may be produced. If a ligature is tied loosely around the egg at the first cleavage exactly along the division plane between the first two blastomeres, it will be found later that the long axis of the single embryo lies, in the great majority of cases, across the ligature, and only in a small percentage of cases does the median plane correspond with that of the ligature, and, therefore, with the first cleavage plane. If one of the latter eggs is allowed to develop to the blastula stage, and the ligature is then drawn tighter, so that the blastula is completely constricted, an embryo develops from each half. If one of the former eggs is allowed to develop to a stage when the medullary plate is laid down, but is not yet sharply marked off, and the ligature is then tightened, there will be formed (the plane of constriction being across the medullary plate) from the anterior part a normal head with eyes, nasal pits, ears, and a piece of the notochord, and from the posterior part there will be formed, at its anterior end, another new head just behind the ligature. Ear-vesicles develop in this part at the typical distance from the anterior end. The brain that develops has a typical cervical curvature, and eye evaginations appear at the anterior end. The chorda, that extended at first to the anterior end of this region, is partially absorbed.