from the frogs produced by fertilization with a spermatozoön. This makes the proof conclusive that the methods of artificial parthenogenesis can result in the production of normal organisms which can reach the adult stage. Bancroft and the writer tried to determine the sex of a parthenogenetic tadpole and of a frog just carried through metamorphosis. Since in early life the sex glands of both sexes in the frog contain eggs it is not quite easy to determine the sex, except that in the male the eggs gradually disappear and from this and other criteria we came to the conclusion that both parthenogenetic specimens, which were four months old, were males. The writer has recently examined the gonads of a ten months old parthenogenetic frog. Here no doubt concerning the sex was possible since the gonads were well-developed testicles containing a large number of spermatozoa of normal appearance, and no eggs.[114] (Figs. 7 and 8.) This would indicate that the frog belongs to those animals in which the male is heterozygous for sex. [114] Since this was written, two more of the parthenogenetic frogs over a year old died. Both were males. 8. The fact that the egg of so high a form as the frog can be made to develop into a perfect and normal animal without a spermatozoön--although normally the egg of this form does not develop unless a spermatozoön enters--corroborates the idea expressed in previous chapters that the egg is the future embryo and animal; and that the spermatozoön, aside from its activating effect, only transmits Mendelian characters to the egg. The question arises: Is it possible to cause a spermatozoön to develop into an embryo? The idea has been expressed that the egg was only the nutritive medium on which the spermatozoön developed into an embryo, but this idea has been rendered untenable by the experiments on artificial parthenogenesis. Nevertheless the question whether or not the spermatozoön can develop into an embryo on a suitable culture medium remains, and it can only be decided by direct experiments. It was shown by Boveri, Morgan, Delage, Godlewski, and others, that if a spermatozoön enters an enucleated egg or piece of egg it can develop into an embryo, but since the cytoplasm of the egg is the future embryo this experiment proves only that the egg nucleus may be replaced by the sperm nucleus; and also that the sperm nucleus carries into the egg, the substances which induce development. Incidentally these experiments on merogony also prove that the mere mechanical tearing of the cortical layer,--which must happen in the separation of the unfertilized egg into parts with and without a nucleus,--by dissection or by shaking, is not sufficient to start development in the sea-urchin egg.