The noteworthy thing about this “chromosome x” is the fact that in every essential detail it corresponds to the accessory chromosome of the Orthoptera. It is a spermatogonial chromosome that comes over intact into the spermatocyte; it retains its form and staining power unchanged through the prophase of the spermatocyte; it divides in only one of the spermatocyte mitoses; and is a large and conspicuous element of the cell at all times. This “chromosome x” agrees just as closely in its description to the accessory chromosome as do the ordinary ones of the two orders, and, if Montgomery’s account is correct, there would seem to be no reason for doubting their identity. In two respects, however, there are differences between these structures. First, it is to be noted that the “chromosome x” divides in the first spermatocyte, while the accessory chromosome undergoes separation in the second spermatocyte. Should Montgomery’s observations prove correct, it would yet indicate no fundamental difference in the character of the element, for the result is the same whether division takes place in the first or second mitosis. In either event, one-half the spermatozoa are provided with the odd chromosome while the remaining half are not. The second point of difference would seem to be a more serious one. Montgomery states that during the spermatogonial mitosis the “chromosome x” regularly divides as do all the other chromosomes, _i. e._, longitudinally. In the spermatocyte mitosis, however, the element is broken across, and the longitudinal split, which is apparent in the early stages, disappears and is not utilized in division. We have here the remarkable occurrence of a chromosome entirely unchanged in its structure, but merely differing in its surroundings, which, instead of dividing along the plane marked out for it, as it has done in all preceding mitoses, breaks across after it is a formed element. An occurrence of this kind, so different from the usual method of division, would require strong proof to establish it, and this, in my opinion, Montgomery has not brought forward.