1. The majority of eggs cannot develop unless they are fertilized, that is to say, unless a spermatozoön enters into the egg. The question arises: How does the spermatozoön cause the egg to develop into a new organism? The spermatozoön is a living organism with a complicated structure and it is impossible to explain the causation of the development of the egg from the structure of the spermatozoön. No progress was possible in this field until ways were found to replace the action of the living spermatozoön by well-known physicochemical agencies.[84] Various observers such as Tichomiroff, R. Hertwig, and T. H. Morgan had found that unfertilized eggs may begin to segment under certain conditions, but such eggs always disintegrated in their experiments without giving rise to larvæ. In 1899 the writer succeeded in causing the unfertilized eggs of the sea urchin _Arbacia_ to develop into swimming larvæ, blastulæ, gastrulæ, and plutei, by treating them with hypertonic sea water of a definite osmotic pressure for about two hours. When such eggs were then put back into normal sea water many segmented and a certain percentage developed into perfectly normal larvæ, blastulæ, gastrulæ, and plutei.[85] Soon afterward this was accomplished by other methods for the unfertilized eggs of a large number of marine animals, such as starfish, molluscs, and annelids. None of these eggs can develop under normal conditions unless a spermatozoön enters. These experiments furnished proof that the activating effect of the spermatozoön upon the egg can be replaced by a purely physicochemical agency.[86] [84] The substitution of well-known physicochemical agencies for the mysterious action of the spermatozoön was the task the writer set himself in this work and not the explanation of natural parthenogenesis, as the author of a recent text-book seems to assume. [85] Loeb, J., _Am. Jour. Physiol._, 1899, iii., 135; 1900, iii., 434. [86] Loeb, J., _Artificial Parthenogenesis and Fertilization_, Chicago, 1913. The reader is referred to this book for the literature on the subject.