The diagnosis of infertility and sterility rests upon a thorough physical examination of the genital organs, together with a detailed study of the semen. The history of the animal and herd involved must also be very carefully inquired into, especially the part covering the result of every service by the sire in question. Besides this, we must always consider all factors which have a bearing upon the subject, remembering the physiology of each part, and the role it plays in reproduction. Bacteria gain entrance to many parts of the tract, where they multiply and probably add toxic products to the seminal fluid, altering its biological character and resulting in partial or total destruction of the secretory tissues. The testes, epididymes, vesicles, prostate, and other parts, each contribute their essential part to the semen, abnormalities of any one of which, as a rule, result in interferences with reproduction. If the vesicles are involved, we must bear in mind just what is the part played by their secretion, and what is the probable result if their essential elements are not added to the semen. In like manner, we must consider the prostate, whose secretion stimulates the vitality of the sperms, and adds fluid bulk to the semen. Extirpation of the vesicles or prostate alone results in lowered fertility, without altering the sexual desire, while removing both glands produces total sterility. Partial or total destruction of the parenchyma of either gland produces the same effect as extirpation, in that its function is altered or entirely absent.