The observations of Williams (19) in a pure bred dairy herd bring out quite clearly the relation of the bull to the dissemination of genital infections. The bulls in this particular herd were abnormal in many respects, as demonstrated by pathological changes in their genital organs, bacterial invasion of the parts, abnormalities of the semen and spermatozoa, and the probable transmission of infection to the females. Several of the sires from this herd furnished much of the material for the early basic work of this investigation. Since then the tracts of other sires have been worked upon with quite similar or identical results. To summarize the work already done, most investigators have considered the bull as merely a mechanical carrier of _Bact. abortum_ infection, though all are more or less suspicious of his ability to become an active spreader. Schroeder, however, states that the organisms are eliminated with the semen, but infection of the female occurs secondarily through the digestive tract by contamination of the food with the semen. Other investigators bring out fundamental points demonstrating the importance of other organisms than the Bang bacillus and call attention to the need of a more thorough study of the anatomy, physiology, and pathology of the male genital tract. Any study of the genital organs must of necessity rest fundamentally upon a thorough knowledge of the anatomy and physiology of those parts. Too few of us have stopped to consider these questions thoroughly, with the result that our ideas on the problem are more or less vague. It is much easier to understand why abnormal spermatozoa occur so frequently, or changes take place in the semen with death or weakening of the germinal elements, if we realize or stop to consider the highly differential process of spermatogenesis, and the various structures which contribute to the formation of the semen. We must come to realize that each part of the genital tract is essential to the normal functioning of the whole, and that the genital tract and reproduction are in turn dependent upon the proper functioning of the entire body. Walker (20) emphasizes the importance of a thorough knowledge of physiology in stating, “Although the subject of sterility has attracted the attention of the medical profession; and although much has been written on its causes and treatment, it cannot be claimed that the practical results obtained up to the present time are satisfactory, or that when consulted for sterility, the medical man of today can hold out to his patient much more hope of successful treatment than the medical man of fifty years ago. Our failure in this respect is in the main due to an ignorance of the physiology of reproduction.” To bring out some of these points, the anatomy of the tract will be reviewed briefly, together with the physiology of reproduction, and the various factors which should be considered in a study of the problem.