In the artificial breeding of trout, which is conducted under cover, in which process the embryos which have crept out of the ovarian follicle are kept with the yolk-sacs in small reservoirs under a continuous flow of water, it is observed that single individuals develop themselves further. They lose their store of yolk with the yolk-sacs. In the course of their further development and nutrition they arrive only very slowly at the development of the internal generative organs. Even in a very advanced stage the sex is not yet so plainly indicated as in fish of the same size living free. Indeed, it is even affirmed by many persons of experience that in the artificial breeding of trout even those that have attained their full growth remain unfruitful and cannot be used for further breeding. (D’Audeville and Arens raised, in the case of trout, more females by dry impregnation.) * * * * * Here might be adduced also many other doctrines of greater or less interest respecting the theory of the origin of sexual distinction. Only in order that I may not introduce too much literary matter, I shall mention only a few of the more important and noticeable theories before I return once more to the experiments upon the influence of food upon the development of sex. Janke’s work (small edition), published at Stuttgart in 1896, furnishes a synopsis of the literary work done, as do other books which treat of this subject. * * * * * Mons. Thury, professor at Geneva, published at Leipzig, in the year 1863, a book on the law of breeding the sexes, which, on account of its contents, attracted great attention. In this work the author, after a number of successful experiments and other investigations, shows how an influence may be exerted over the sex of plants, animals, and men. This work stirred me up to the endeavor to devote myself to this question, so far as that might be possible. I shall give a short sketch of Thury’s work, together with the critical revision of it by Dr. H. A. Pagenstecher, of Heidelberg. The doctrine respecting the origin of sex in cattle was laid down by Thury from his own investigations. The principal point in his doctrine of the origin of sex in animals he considers to be the condition of the ovum at the time when it is fertilized. If the ovum has reached the stage of ripeness, which may be described as an advanced stage, we may expect to have, after fertilization has taken place, a male individual, which will develop itself out of the ovum. If, however, the ovum has reached only an imperfect state of ripeness when successful impregnation takes place, then no such powerful and perfect specimen of the race as the male is can be developed, and the result of such an ovum is always a female. * * * * *