Differentiation among Chromosomes in Elodea

Abstract

1. The pollen mother cells, before rounding off, have dense, finely granular cytoplasm with a large nucleus. 2. The resting nucleus consists of delicate chromatin threads or meshes, with some nodelike structures at the intersections of the threads. 3. The first indication that reduction is about to take place is the appearance of a clear space at one side of the nucleus, the thickening of the chromatin threads, and the enlargement of the nodes of the chromatin network. 4. During synizesis there is growth of the nucleus and a considerable contraction of the reticulum. The threads become rear-ranged, and a double continuous filament is formed, which has a granular character. 5. After synizesis the filament, commonly known as the spireme, loosens from a tight knot and extends throughout the nuclear cavity. Beautiful loops are formed, which are gradually detached from the base. The spireme becomes a thickened and smooth filament before the second contraction and segmentation take place. 6. The individual segments unite end to end and gradually condense, sometimes twisting about each other. The method of reduction in Elodea, therefore, is telosynaptic. 7. The bivalent chromosomes are twenty-four in number, one pair of which is conspicuously longer and thicker than the others, and another pair is characterized by having unequal members. 8. The chromosomes become shortened and thickened by contraction, while the nuclear membrane becomes thinner and thinner, and finally disappears. This disappearance of the nuclear membrane is followed by the formation of the heterotypic spindle, which draws the chromosomes into the equatorial plate. 9. Each of the bivalent chromosomes is attached to the spindle by one end, and the two constituent chromosomes move apart. The giant (L) chromosome splits lengthwise and forms two large daughter chromosomes; while the small (Y) chromosome separates from the other member of the pair and goes to one pole. 10. As the daughter chromosomes move toward the poles they become V-shaped, with the apex of the V attached to the spindle. Their double nature is sometimes shown at their tips. 11. At the pole of the heterotypic spindle the daughter chromosomes unite side by side, and cross connections appear. 12. The resulting daughter nuclei from heterotypic division do not pass into a true resting stage, not reaching a true reticulum, but soon reorganizing for the next mitosis. As the nuclear membranes disappear the two homotypic spindles appear simultaneously. The chromosomes arrange themselves in the equatorial plate and then split equally lengthwise. 13. Two of the four resulting nuclei receive one giant (L), one small (Y) chromosome, and twenty-two of the chromosomes which are now becoming designated as autosomes; while each of the other two receives one giant (L) chromosome, the larger member of the unequal pair (X), and twenty-two autosomes. Thus, in the reconstruction of the four daughter nuclei, two of them contain more chromatin than the other two. The resulting pollen grains, however, look alike. 14. In the somatic mitosis there are forty-eight chromosomes, two of which are conspicuously longer and thicker than the others, while one is noticeably smaller. In the reduction, the two large chromosomes unite with each other, and the small chromosome unites with one of ordinary size.