SPERMATOGENESIS OF THE MYRIOPODS

Abstract

The chromosome group of the primary spermatocytes of Scolopendra heros is made up of sixteen bivalent chromosomes (tetrads) and one univalent chromosome (dyad), the accessory chromosome. The chromosomes show such constancy in shape in the prophase and metaphase of the primary spermatocytes, and in their relation to the mantle fibers of the first maturation spindle, that they seem naturally to group themselves under four distinct types. These may be designated respectively, as the cross-shaped tetrads, the double-V-shaped tetrads, the rod-shaped tetrads, and a single-rod-shaped dyad. The cross-shaped tetrads are six in number and may be arranged in a graded series as regards size, the difference in bulk being sufficiently great to allow the individual chromosomes of this type to be distinguished. One of the chromosomes of this type (the largest one) can furthermore often be identified by its tendency to lag behind the others during the early metaphase. Five of the tetrads are of the double-V shape. The individuals of this type also may be distinguished by differences in bulk. The rod-shaped tetrads are present to the number of five. These show constant size relations and may readily be arranged in a graded series as regards magnitude. One of the tetrads of this type differs from the others in the form it assumes during actual division. It seems to divide transversely, while the others are dividing longitudinally. The accessory chromosome is univalent and passes to one of the secondary spermatocytes without division. During the metaphase it is connected by mantle fibers to only one pole of the spindle. As a result of the first spermatocyte mitosis fifteen of the tetrads are divided longitudinally (equationally), while the one remaining tetrad divides transversely (reductionally). The failure of the accessory chromosome to divide is, also, in effect a reductional division. During the later stages of the first maturation division and during the metaphase of the second spermatocyte, it is possible to distinguish the daughter chromosomes derived from the several types of tetrads, by their shape and their relations to the mantle fibers. The individuals of the various types show the same size ratio as exists between the chromosomes of the first spermatocyte, although, of course, the actual difference in bulk is but half as great. The above results seem to establish as a fact, or at least as a very strong probability, that the chromosomes of Scolopendra heros are distinct and definite individuals, which, under similar circumstances, i. e., in the same cell generation, show a remarkable constancy in form, relative size, and in their attachment to the mantle fibers. This constancy of form, size and behavior, affords a strong argument in favor of the theory of the individuality of the chromosomes in this species in particular and adds support to the evidence derived from the study of other forms, to the general application of the theory.