A large part of Loeb's book is devoted to a description of the author's remarkable experiments in artificial parthenogenesis, and an attempt to show that they offer a complete explanation. Sir William Tilden, one of the greatest living authorities on organic chemistry, tells us that "too much has been made of the curious observations of J. Loeb and others"; and he definitely states that when we consider "the propagation of the animal races by the sexual process ... there can be no fear of contradiction in the statement that in the whole range of physical and chemical phenomena there is no ground for even a suggestion of an explanation." Behind this pronouncement of an expert, one might well shelter oneself; but the question under consideration merits a little further treatment. The reproduction of kind, though usually a bi-sexual process, may, however, normally in rare cases be uni-sexual, and this process is known as Parthenogenesis. Even in human beings certain tumours of the sex-glands, known as teratomata, very rare in women and even rarer, if ever existent, in men, have been claimed as examples of attempts at parthenogenesis, and so far no better explanation is available. Now Loeb and others have succeeded in certain forms--even in a vertebrate like the frog--in inducing development in unimpregnated ova. The evidence for all these things is still slender; but we will content ourselves with noting that point and passing on to the consideration of the phenomena and the claims put forward in connection with them. We find the task of unravelling the writer's meaning rendered more difficult by a certain confusion in his use of terms, since fertilisation, _i.e._ syngamy--the union of the different sex products--seems to be confused with segmentation, _i.e._ germination; and this confusion is accentuated by the claim that "the main effect of the spermatozoon in inducing the development of the egg consists in an alteration in the surface of the latter which is apparently of the nature of a cytolysis of the cortical layer. Anything that causes this alteration without endangering the rest of the egg may induce its development." When the spermatozoon enters the ovum it causes some alteration in the surface membrane of the latter which, amongst other things, prevents the entrance of further spermatozoa. Loeb thinks that in causing this alteration it sets up the segmentation of the ovum. That there is a close connection between the two events seems undoubted; that they are in relation of cause and effect seems likely. It is quite evident that an artificial stimulus can in certain cases set up segmentation, but never can it cause the fertilisation of the ovum. It may very likely produce the same change in the membrane that is caused by the entrance of the spermatozoon under normal circumstances--membrane formation may be necessarily coincident with the liberation in the egg of some zymose which arises from a pre-existent zymogen. But we are