No fact is more evident in practice than that the differential diagnosis of the cause of sterility in a given "mating" demands prolonged and accurate study to determine which of the many factors concerned1is at fault in the given case. Increasing experience has, moreover, made it evident that the rough clinical methods which I have hitherto used were inadequate for the accurate establishment of such a diagnosis, and has led me to undertake during the last year a somewhat extensive search for more exact methods. The results obtained are already far too voluminous for presentation in any single paper, more especially under the time limitations imposed at this meeting. The subjects which I mean to take up here in brief abstract are, first, the determination of the original vitality of the spermatozoa deposited by a given male; second, the conditions which permit or prevent effective ovulation; third, the