1. Thirty-seven chromosomes differing somewhat in size occur in the spermatogonia. One, the accessory, is distinctly larger than the others. 2. In the spermatogonial division the accessory divides a little in advance of the other chromosomes. 3. Nineteen chromosomes appear in the primary spermatocyte division, of which eighteen are evidently bivalent and the other is the accessory. 4. In the secondary spermatocyte division the heart-shaped accessory passes undivided to one pole in advance of the other chromosomes. 5. The primary spermatocyte division is evidently the reduction division, giving rise to two different types of secondary spermatocytes, one with the accessory and the other lacking it. 6. There is no resting stage following the first spermatocyte division. 7. A second pairing of the chromosomes takes place so that only one-fourth the original number of chromosomes appear for division in the secondary spermatocyte. 8. The accessory chromosome divides in the secondary spermatocyte division a little in advance of the other chromosomes the same as it does in the spermatogonia. 9. The one type of secondary spermatocyte, which contains the accessory, gives rise to two spermatids, each containing the accessory and nine bivalent chromosomes. 10. The other type of secondary spermatocyte, which lacks the accessory, gives rise to two spermatids, each containing only the nine bivalent chromosomes. 11. In terms of univalence, then, one type of spermatid receives eighteen chromosomes plus the accessory and the other type receives only the eighteen ordinary chromosomes. 12. In view of the foregoing facts, two different types of spermatozoa, equal in numbers, are produced in the horse; the one type contains in addition to the ordinary chromosomes the accessory, and is apparently the female determining spermatozoön. 13. Actual measurements of six hundred mature spermatozoa reveal the interesting fact that two distinct types of spermatozoa as regards size are produced, the one being much larger and presumably the one which bears the accessory chromosome. 14. The dimorphic condition among the spermatozoa of the horse lends additional support to the chromosome theory of sex determination. 15. The developing spermatozoa invariably cast off a mass of cytoplasm. 16. A chromatoid body, which simulates the appearance of a y-element in the primary spermatocyte division stages, makes its appearance during the growth period and can be traced forward until it is finally thrown off with the ball of cytoplasm in the developing spermatozoön. It does not contribute in any visible way to the formation of the spermatozoön.