One factor we must bear in mind in the study of the semen obtained from the vagina, is that the spermatozoa may be highly motile before ejaculation, but the admixture of hostile vaginal mucus may inhibit or destroy the motility. On the other hand, the conditions may be reversed. Cary (35), in one instance, found that the spermatozoa in a sample of semen collected from a condom, appeared to be of very low vitality, while when they were mixed with the vaginal secretions, an exaggerated activity was manifested. May we not have to contend with this factor in some herds in which there is a very distinct acid and toxic vaginal secretion from the products of cervicitis and vaginitis? In a study of motility, we must consider not only the abnormal types which may be encountered but the percentage of motile cells, and the duration of the movement. In _necrospermia_ all the ejaculated cells are motionless or dead. In other specimens, varying percentages of the cells are without motion, and the others may be possessed of full and lasting motility. On the other hand, the motility in some cases is very active at first, but quickly subsides even under the best of conditions. The appearance in freshly ejaculated semen of numerous sperms that have a tendency to take on the “undulatory tactile” type of motion when they should be in a highly active state, is very indicative of lowered vitality. Many specimens present this very picture, whereas the very active progressive movement should, under proper conditions, survive for a considerable time before it gives way to the second, and slower type. The cells frequently early bunt into epithelial cells or clumps of immotile sperms, then back out and move around sluggishly, only to repeat the same performance till they stop moving entirely. I have seen one specimen in which the sperms all tended to clump. Whether this was the result of some agglutinative substance in the vaginal secretion is problematical. I have seen several specimens of semen in which practically all the sperms were motile when first examined, but the motion did not survive for any great length of time. Even a small percentage of motionless sperms or of those showing lowered vitality is a considerable factor in potency. Although millions of the germinal elements are ejaculated into the vagina, large numbers of them are destroyed or become motionless there, and a small number is left behind in the cervix and uterus; so that even though but a single sperm is required for fertilization, the chances of impregnation are diminished in proportion to the number of dead or defective sperms.