From the very minute size of the human ovum, and the difficulty of detecting it, the existence of this little corpuscule was not satisfactorily ascertained until modern times. Although De Graaf had observed ova in the Fallopian tube so early as 1668, which fact had been confirmed by the researches of Dr. Haighton and Mr. Cruickshank, still, as no traces of such ova had been discovered in the Graafian vesicle, and as it was evident that the Graafian vesicle, from its size, &c. could not pass along the Fallopian tube, it was concluded that the inner surface of the vesicle was a species of glandular structure which secreted the fluid with which it was filled, and which was analogous to the semen of the male testicle; hence, in former times, the ovaries were known by the name of _testes muliebres_. The celebrated anatomist Steno[2] first pointed out the analogy between these organs and the ovaries of the fish tribe: this view was afterwards supported by De Graaf,[3] and they have since continued to retain the name of ovaries. To Professor von Baer, now of St. Petersburg, is due the merit of having first pointed out the distance of the ovum in the Graafian vesicle, and of thus putting beyond all doubt the accuracy of De Graaf's observations, as well as those of Dr. Haighton and Mr. Cruickshank. _Corpus luteum._ Upon impregnation taking place, one or more of the most prominent Graafian vesicles begins to show marks of considerable vascularity, both in its external capsule and in the surrounding stroma of the ovary. The vesicle swells, and at length bursts, discharging its contents into the funnel-shaped extremity of the Fallopian tube, which firmly grasps the ovary at this point by means of its fimbriæ. These changes begin to take place immediately after impregnation; the inner lining of the vesicle, which Professor von Baer considers to be a mucous membrane, appears to undergo a rapid development, much more so than the external capsule which contains it. It is, therefore, thrown into a number of corrugations by which the cavity of the vesicle is greatly diminished; it becomes much thicker, and assumes a yellow colour. As its growth proceeds, the cavity of the vesicle becomes still farther contracted, until being unable longer to retain its contents, it bursts and discharges them as above described. [Illustration: Corrugation of the lining membrane of the Graafian capsule after impregnation. _From Baer._] The remains of the ruptured vesicle form a round glandular yellow coloured body, called _corpus luteum_: it projects considerably from the surface of the ovary, attaining the size of a small mulberry. In the middle of this projection there is a little irregular and generally triangular depression or indentation, which is the opening through which the ovum was discharged from the Graafian vesicle: this after a short time closes, forming a little cicatrix on the surface of the ovary.