7. When the experiments on artificial parthenogenesis were first published they aroused a good deal of antagonism not only among reactionaries in general but also among a certain group of biologists. O. Hertwig had defined fertilization as consisting in the fusion of two nuclei, the egg nucleus and the sperm nucleus. No such fusion of two nuclei takes place in artificial parthenogenesis since no spermatozoön enters the egg, and it became necessary, therefore, to abandon Hertwig’s definition as wrong. The objection raised that the phenomena are limited to a few species soon became untenable since it has been possible to produce artificial parthenogenesis in the egg of plants (_Fucus_, according to Overton) as well as of animals, from echinoderms up to the frog; and it may possibly one day be accomplished also in warm-blooded animals. A second objection was that the eggs caused to develop by the methods of artificial parthenogenesis could never reach the adult stage and that hence the phenomenon was merely pathological. There was no basis for such a statement, except that it is extremely difficult to raise marine invertebrates. Delage[113] was courageous enough to make an attempt to raise parthenogenetic larvæ of the sea urchin beyond the larval stage and he succeeded in one case in carrying the animal to the mature stage. It proved to be a male. [113] Delage, Y., _Compt. rend. Acad. Sc._, 1909, cxlviii., 453. Better opportunities were offered when a method was discovered which induced the development of the unfertilized eggs of the frog. In 1907, Guyer made the surprising observation that if he injected lymph or blood into the unfertilized eggs of frogs he succeeded in starting development and he even obtained two free-swimming tadpoles. “Apparently the white rather than the red corpuscles are the stimulating agents which bring about development, because injections of lymph which contains only white corpuscles produce the same effects as injections of blood.” Curiously enough, Guyer thought that probably the cells which he introduced and not the egg were developing. In 1910, Bataillon showed that a mere puncture of the egg with a needle could induce development but he believes that for the full development the introduction of a fragment of a leucocyte is required. Bataillon has called attention to the analogy with the writer’s results on lower forms, the puncturing of the egg corresponding to the cytolysis of the surface layer of the egg and the introduction of a leucocyte as the analogue of the second or corrective factor. The method of producing artificial parthenogenesis by puncturing the egg has thus far been successful only in the egg of the frog. The writer has tried it in vain on the eggs of many other forms. He has at present seven parthenogenetic frogs over a year old, produced by merely puncturing the eggs with a fine needle (Fig. 6). These frogs have reached over half the size of the adult frog. They can in no way be distinguished