A Graafian vesicle cannot be converted into a corpus luteum except by actual and effective sexual intercourse; and the strange and discrepant accounts which have every now and then been published, even by authors of considerable repute, of corpora lutea having been found in the ovaries of virgin and even newly-born animals merely prove that the true characteristics of the corpus luteum were not sufficiently known. The irregular cysts, cavities, or deposites of whitish or yellowish structure which are frequently found in the ovary, independent of impregnation, and which have been improperly enough called virgin corpora lutea, present points of difference so marked that they can scarcely be mistaken by an experienced eye. The angular cavity opening externally, the stellated, radiated, cicatrix-like appearance, which a section of the corpus luteum presents, its soft and delicate structure as described by Dr. Hunter, and above all its vascularity, and the facility with which its vessels can be injected from the general tissue of the ovary, are characters only found in a true corpus luteum. Virgin corpora lutea frequently occur under circumstances of disease, especially those of a tubercular character. They frequently appear as distinct cysts, the walls of which are semi-cartilagenous; at other times they seem to be nothing more than a coagulum of blood: they seldom project much from the ovary, and in no instance have they the peculiar structure of the corpus luteum, nor the external cicatrix, nor are they capable of being injected. After awhile the cavity of the corpus luteum contracts, and the opening into it closes. The surrounding stroma loses its vascularity, the prominence at this part of the ovary gradually subsides, and the ovary returns to its former size. The periods at which these changes take place vary, but with the exception of those first mentioned they proceed slowly whilst pregnancy lasts, after which time, now that the increased activity of the pelvic circulation peculiar to that period has ceased, they advance more rapidly. [Illustration: Corpus luteum at the end of the ninth month. _From Dr. Montgomery._]