It did not therefore appear likely that a few treads, in the beginning of the year, should suffice to render fertile the whole of the eggs that are to be laid in its course. Upon one occasion, however, in the spring season, by way of helping out Fabricius, and that I might have some certain data as to the time during which the fecundating influence of intercourse would continue, and the necessity of renewed communication, I had a couple of hens separated from the cock for four days, each of which laid three eggs, all of which were prolific. Another hen was secluded, and the egg she laid on the tenth day afterwards was fruitful. The egg which another laid on the twentieth day of her seclusion also produced a chick. It would therefore seem that intercourse, once or twice repeated, suffices to impregnate the whole bunch of yelks, the whole of the eggs that will be laid during a certain season. I shall here relate another observation which I made at this time. When I returned two of the hens, which I had secluded for a time, to the cock, one of which was big with egg, the other having but just laid, the cock immediately ran to the latter and trod her greedily three or four times; the former he went round and round, tripping himself with his wing and seeming to salute her, and wish her joy of her return; but he soon returned to the other and trod her again and again, even compelling her to submit; the one big with egg, however, he always speedily forsook, and never solicited her to his pleasure. I wondered with myself by what signs he knew that intercourse would advantage one of these hens and prove unavailing to the other. But indeed it is not easy at any time to understand how male animals, even from a distance, know which females are in season and desirous of their company; whether it be by sight, or hearing, or smell, it is difficult to say. Some on merely hearing the voice of the female, or smelling at the place where she has made water, or even the ground over which she has passed, are straightway seized with desire and set off in pursuit to gratify it. But I shall have more to say on this subject in my treatise on the Loves, Lusts, and Sexual Acts of Animals. I return to the matter we have in hand. EXERCISE THE SEVENTH. _Of the abdomen of the common fowl and of other birds._ From the external orifice proceeding through the vulva we come to the uterus of the fowl, in which the egg is perfected, surrounded with the white and covered with its shell. But before speaking of the situation and connections of this part it seems necessary to premise a few words on the particular anatomy of the abdomen of birds. For I have observed that the stomach, intestines, and other viscera of the feathered kinds were otherwise placed in the abdomen, and differently constituted, than they are in quadrupeds.