After having given ourselves the trouble to glance through the long series of different views which have existed concerning the origin of sex, we are struck with the fact, that scarcely a single objective observation has been made that could lead to a positive result in the development of sex in the new-born. Out of all these theories, however, one hypothesis does seem to be tenable, and to that we shall find ourselves compelled to give our adhesion here, after having set forth the detailed explanations which shall presently follow. Now, we were not led to the recognition of the truth of this teaching by previously assumed theories. On the contrary, it was after we had completed our experiments, and had been led by them to positive results, that we found ourselves compelled, in order to find an explanation of the whole process, to fall back upon the theory of cross-heredity of sex, and to place it, by our own experience, upon firmer foundations. It must be briefly remarked here that the literature of this subject is very extensive, and to trace out all the literary results would lead us too far. For which reason I have limited myself to the actual facts. * * * * * In order to enter more fully into our theory, it will in the first place be necessary to turn our attention to the products of excretion which are eliminated from the bodies of animals, in a more or less solid, fluid, or gaseous form, as the results of the transformations of matter which have taken place in the bodies of the animals. The chemical constituents of the evacuations are either such as it has not been possible to convert to use in the body, or such as are evacuated as the final product of the oxidation which has been effected in the body by the process of combustion. Of these the former are evacuated from the body as so much inert matter which has not been affected by the digestive juice, nor altered in any other way. The latter are given off from the animal’s body in various states of oxidation in the urine, sweat, or dung, or else in a gaseous form by the lungs. A great number of the substances which have been hitherto exactly examined are secreted by the kidneys and appear in the urine. In this fluid are found nitrogenous products of secretion, others free from nitrogen, and inorganic substances. Amongst the substances free from nitrogen, I found myself prompted _more particularly to select as an object of my attention the carbo-hydrate (sugar) found in the urine_. Three groups of closely connected compounds are reckoned amongst the carbo-hydrates. They consist of carbon, hydrogen, and oxygen, which contain in the molecule six, or a multiple of six, atoms of carbon. The hydrogen and oxygen are in the same proportion as in water. (Arnold.)