1. Twenty-two chromosomes differing considerably in size occur in all spermatogonia in which a definite count could be made. In a few instances two, apparently the two accessory chromosomes, were seen considerably to one side of the main mass of chromosomes, surrounded by a small clear court of cytoplasm.2. Twelve chromosomes appear for division in the primary spermatocyte, of which ten are evidently bivalent and two accessories.3. The two accessory chromosomes pass undivided to one pole of the spindle considerably in advance of the other chromosomes, with the result that half of the daughter cells in this division receive twelve, and half, only ten univalent chromosomes. This is evidently the reduction division.4. The ten univalent chromosomes which passed to the one secondary spermatocyte unite again in pairs, at least in the majority of cases, to form five bivalent chromosomes which appear at the equator of the spindle when the cell is ready for division. The division here is presumably an equation and not a second reduction division, judging from the size, shape and general appearance of the, resulting daughter chromosomes. Thus while each of the spermatids formed as a result of this division receive only five chomosomes, the latter are bivalent and equivalent to ten of the somatic or spermatogonial chromosomes. There is some slight evidence that the secondary spermatocytes may occasionally divide with these chromosomes in their original condition of univalence.5. Ten of the twelve chromosomes which passed to the other pole of the spindle in the primary spermatocyte behave in precisely the same way as described in the last paragraph. The two accessory chromosomes come to the equator of the spindle in the secondary spermatocyte with the five bivalents thus making in all seven. Each accessory now divides so that the resulting spermatids each receive seven chromosomes; that is, five bivalent plus two accessory, or the equivalent of twelve univalent chromosomes.6. In reality, then, of the total number of spermatids, half have in all probability received ten, and half, twelve (10 plus 2) univalent chromosomes. Inasmuch as the spermatids transform directly into spermatozoa, there must be two classes of the latter differing with respect to whether they have or do not have the two accessory chromosomes.7. It is a significant fact that approximately half the resting spermatids when strongly decolorized after iron-hæmatoxylin staining, show two chromatin nucleoli and half do not. It seems probable that these nucleoli may correspond to the accessory chromosomes and are to be identified with the two nucleoli of the primary spermtocyte and the two eccentric chosomes seen in the spermatogonia.8. It is probable that in man and certain other vertebrates, as in the insects, myriapods and arachnids, the accessory chromosomes are in some way associated with the determination of sex.