definition of the sexual impulse. "The genesic need may be considered," writes Féré, "as a need of evacuation; the choice is determined by the excitations which render the evacuation more agreeable."[3] Certain facts observed in the lower animals tend to support this view; it is, therefore, necessary, in the first place, to set forth the main results of observation on this matter. Spallanzani had shown how the male frog during coitus will undergo the most horrible mutilations, even decapitation, and yet resolutely continue the act of intercourse, which lasts from four to ten days, sitting on the back of the female and firmly clasping her with his forelegs. Goltz confirmed Spallanzani's observations and threw new light on the mechanism of the sexual instinct and the sexual act in the frog. By removing various parts of the female frog Goltz found that every part of the female was attractive to the male at pairing time, and that he was not imposed on when parts of a male were substituted. By removing various of the sense-organs of the male Goltz[4] further found that it was not by any special organ, but by the whole of his sensitive system, that this activity was set in action. If, however, the skin of the arms and of the breast between was removed, no embrace took place; so that the sexual sensations seemed to be exerted through this apparatus. When the testicles were removed the embrace still took place. It could scarcely be said that these observations demonstrated, or in any way indicated, that the sexual impulse is dependent on the need of evacuation. Professor Tarchanoff, of St. Petersburg, however, made an experiment which seemed to be crucial. He took several hundred frogs (_Rana temporaria_), nearly all in the act of coitus, and in the first place repeated Goltz's experiments. He removed the heart; but this led to no direct or indirect stoppage of coitus, nor did removal of the lungs, parts of the liver, the spleen, the intestines, the stomach, or the kidneys. In the same way even careful removal of both testicles had no result. But on removing the seminal receptacles coitus was immediately or very shortly stopped, and not renewed. Thus, Tarchanoff concluded that in frogs, and possibly therefore in mammals, the seminal receptacles are the starting-point of the centripetal impulse which by reflex action sets in motion the complicated apparatus of sexual activity.[5] A few years later the question was again taken up by Steinach, of Prague. Granting that Tarchanoff's experiments are reliable as regards the frog, Steinach points out that we may still ask whether in mammals the integrity of the seminal receptacles is bound up with the preservation of sexual excitability. This cannot be taken for granted, nor can we assume that the seminal receptacles of the frog are homologous with the seminal vesicles of mammals. In order to test the question, Steinach chose the white rat, as possessing large seminal