The Spermatogenesis of Stenobothrus viridulus; with Special Reference to the Heterotropic Chromosome as a Sex Determinant in Grasshoppers.

Abstract

Summary 1. The apical cell is found at the extreme anterior end of the mature follicle, completely surrounded by a single layer of primary spermatogonial cells. There is only one apical cell in each follicle. 2. The secondary spermatogonia occur in clusters and morphologically appear similar to the primary cells, but are recognizable by the absence of the apical cell and by the number of clusters. 3. The nucleus exhibits an apparently complete reticulum in the resting-stages of both primary and secondary spermatogonia there is no trace of the identity of either heterotropic or ordinary chromosomes. 4. The chromosomes of the spermatogonial complex can be arranged in a graduated series of pairs, and are divisible into three groups, viz., large, small, and medium-sized chromosomes. The number of chromosomes is constant and is seventeen, the fourth largest being unpaired and corresponding with the “monosome” and “accessory” chromosome of other writers. 5. All the members of the spermatogonial complex divide in mitosis; but the odd or heterotropic chromosome often “lags,” and can be seen on the spindle when the ordinary chromosomes are assembling at the poles. 6. The nucleus is at its smallest size after the last spermatogonial division, and this stage is followed by a clearly observable growth-period, extending to the prophase of the first maturation division. In this resting-stage the nucleus again exhibits a chromatin reticulum, the granules being disposed along linin threads the identity of the ordinary chromosomes is lost, but the heterotropic chromosome remains as a dark and homogeneous body close to the periphery of the nucleus, and undergoes no resolution into a spireme. 7. I have found no trace of separate sacs or vesicles in which chromosomes undergo transformation into spiremes, either in the case of the heterotropic or the ordinary chromosomes. 8. The equatorial plate of the primary spermatocyte mitosis shows nine chromosomes, again divisible into three groups as regards size. The fourth largest of the complex is undoubtedly the heterotropic chromosome. The distinct correspondence between the size and shape relationships of the secondary spermatogonial and primary spermatocyte complexes points to the possibility of a lateral conjugation of members of the spermatogonial pairs during the intervening period, but is not a proof of it. 9. The ordinary chromosomes divide in the primary spermatocyte metaphase, and their halves pass to opposite poles of the spindle the heterotropic chromosome shows no sign of division, and passes entire to one daughter cell, while the ordinary chromosomes are still on the equatorial plate. In this manner dimorphism of the subsequent spermatozoa is effected. 10. I have been unable to discover whether reduction—the separation of conjugant members—occurs at the first maturation division or at the next; possibly both divisions are equational, and only a numerical reduction takes place as a result of lateral association of chromatin granules or masses on the reticulum threads prior to this primary spermatocyte prophase of mitosis. 11. There is no resting-stage between the first and second maturation divisions the constriction of the cytoplasm to form the two daughter secondary spermatocytes is closely followed by the appearance of the next karyokinetic spindle. 12. The complex of the secondary spermatocyte cell shows nine or eight chromosomes, and this difference is due to the presence or absence of the heterotropic chromosome, which is found in only 50% of the cells. The chromosomes exhibit the same size relationships that occurred in the previous metaphases, there being three large chromosomes, three small, and two of intermediate size. The heterotropic chromosome, when present, is the fourth largest of the complex. Two of the three small chromosomes are spherical, and the third is ovoid or slightly dumb-bell shaped the remainder appear as two arms jointed at one end and closely apposed to one another. 13. I find no direct evidence to prove that these arms are the representatives of the spermatogonial pairs. 14. Division occurs at the junction of the component arms. The heterotropic chromosome usually “lags,” and can be seen on the spindle when the ordinary chromosomes have passed to the two opposite poles. 15. The formation of the nuclear membrane in the spermatids is followed by resolution of the ordinary chromosomes into their component granules. This process continues until the nucleus appears of an uniformly grey colour, in which the individuality of the chromatin particles is lost. The heterotropic chromosome remains at first as a darkly staining and irregular body, but later undergoes resolution into particles, whose identity is indistinguishable in the common chromatin mass. 16. The appearance of the “centrosome” is followed by the formation of the axial filament, arising near the constriction in the middle of this body. The cytoplasm in this region elongates to form the tail of the unripe spermatozoon, and the axial filament appears as a faint line running down the centre of the tail. The elongation of the nucleus and the tailpiece continues, and is accompanied by a reduction of the “centrosome,” which finally becomes extremely small. 17. The head of the spermatozoon is composed of the nucleus; the “centrosome” forms the middle piece, and the axial filament and its surrounding cytoplasm form the tail. 18. The spermatids travel later towards the posterior end of the follicle with their heads turned towards the anterior end and this phenomenon is observable in the spermatozoa. The spermatids are found scattered in the follicle, the unripe spermatozoa in more closely associated clusters, and the ripe spermatozoa in solid bunches. 19. At no stage have I observed a discharge of chromatin from the nucleus, and I have seen nothing to suggest that the whole of the chromatin is not directly concerned with the transformation from the resting reticulum to the compact chromosome condition of the metaphase. 20. The extreme posterior end of the follicle contains numerous degenerating cells, in which irregularly shaped masses of chromatin stain deeply with the iron haematoxylin. 21. Although the individuality of the chromosomes is completely lost in the resting-stages of the spermatogonia, spermatocytes, and spermatids, with the one exception of the heterotropic chromosome in the primary spermatocyte growth-period, there is strong reason for supposing that the same elements appear on the successive mitotic spindles throughout development. It is possible that the component granules of a particular chromosome are not the same in these cases, for an exchange of chromatin particles may occur during the reticulum stages, and if this occurs we have at present no means of discovering the extent of this exchange. It must, therefore, not be assumed that corresponding chromosomes of two successive metaphases contain the same individual chromomeres.