Page:EB1911 - Volume 21.djvu/799

Rh spindle-fibres in such a way that they form a radiating star-shaped figure—Aster—when seen from the pole of the spindle. This is called the nuclear plate (fig. 2, E, F, G, H). As they pass into this position they undergo a longitudinal splitting by which the chromatin in each chromosome becomes divided into equal halves. (3) Anaphase.—The longitudinal division of the chromosomes is completed by the time they have taken up their position in the nuclear plate, and the halves of the chromosomes then begin to move along the spindle-fibres to opposite poles of the spindle (fig. 2, I, J). Many observers hold the view that the chromosomes are pulled apart by the contraction of the fibres to which they are attached. (4) Telophase.—When they reach the poles the chromosomes group themselves again in the form of stars—Diaster—with spindle-fibres extending between them (fig. 2, K). The chromosomes then fuse together again to form a single thread (fig. 2, L), a nucleolus appears, a nuclear membrane is formed, and daughter nuclei are thus constituted which possess the same structure and staining reactions as the mother nucleus.

The spindle figure is probably the expression of forces which are set up in the cell for the purpose of causing the separation of the daughter chromosomes. Hartog has endeavoured to show that it can only be formed by a dual force, analogous to that of magnetism, the spindle-fibres being comparable to the lines of force in a magnetic field and possibly due to electrical differences in the cell. The spindle arises partly from the cytoplasm, partly from the nucleus, or it may be derived entirely from the nucleus—intranuclear spindle—as occurs in many of the lower plants (Fungi, &c.). The formation of the spindle begins in the prophases of division. A layer of delicate filamentous cytoplasm—kinoplasm—may collect around the nucleus, or at its poles, out of which the spindle is formed. As division proceeds, the filamentous nature of this cytoplasm becomes more prominent and the threads begin either to converge towards the poles of the nucleus, to form a bipolar spindle, or may converge towards, or radiate from, several different points, to form a multipolar spindle. The wall of the nucleus breaks down, and the cytoplasmic spindle-fibres become mixed with those derived from the nuclear network. The formation of the spindle differs in details in different plants.

The significance of this complex series of changes is very largely hypothetical. It is clear, however, that an equal quantitative division and distribution of the chromatin to the daughter cells is brought about; and if, as has been suggested, the chromatin consists of minute particles or units which are the carriers of the hereditary characteristics, the nuclear division also probably results in the equal division and distribution of one half of each of these units to each daughter cell.

Reduction Divisions (Meiosis).—The divisions which take place leading to the formation of the sexual cells show a reduction in the number of chromosomes to one-half. This is a necessary consequence of the fusion of two nuclei in fertilization, unless the chromosomes are to be doubled at each generation. In the vascular cryptograms and phanerogams it takes place in the spore mother cells and the reduced number is found in all the cells of the gametophyte, the full number in those of the sporophyte. We know very little of the details of reduction in the lower plants, but it probably occurs at some stage in the life history of all plants in which sexual nuclear fusion takes place. The reduction is brought about simply by the segmentation of the spirem thread into half the number of segments instead of the normal number. In order to effect this the individual chromosomes must become associated in some way, for there is no diminution in the actual amount of nuclear substance, and this leads to certain modifications in the division which are not seen in the vegetative nuclei. The two divisions of the spore mother cell in which the reduction takes place, follow each other very rapidly and are known as Heterotype and Homotype (Flemming), or according to the terminology of Farmer and Moore (1905) as the meiotic phase. In the heterotype division the spirem thread is divided longitudinally before the segmentation occurs (fig. 2, B), and this is preceded by a peculiar contraction of the thread around the nucleolus which has been termed synapsis (fig. 1, A). A second contraction may take place later, immediately preceding the segmentation of the thread. It has been suggested that synapsis may be connected with the early longitudinal splitting of the thread or with the pairing of the chromosomes, but it is possible

that it may be connected with the transference of nucleolar substance to the nuclear thread. The segments of each chromosome are usually twisted upon each other and may be much contorted (fig. 2, C, D), and appearances are observed which suggest a second longitudinal division, but which are more probably due to a folding of the segment by which the two halves come to lie more or less parallel to each other, and form variously shaped figures of greater or less regularity (fig. 2, E). The chromosomes now become attached to the spindle-fibres (fig. 2, F, G) and as the daughter chromosomes become pulled asunder they often appear more or less V-shaped so that each pair appears as a closed ring of irregular shape, the ends of the V's being in contact thus—<> (fig. 2, H. I, J, K). This V has been variously interpreted. Some observers consider that it represents a longitudinal half of the original segment of the spireme, others that it is a half of the segment produced by transverse division by means of which a true qualitative separation of the chromatin is brought about. The problem is a very difficult one and cannot be regarded as definitely settled, but it is difficult to understand why all this additional complexity in the division of the nucleus should be necessary if the final result is only a quantitative separation of the chromatin. It seems to be fairly well established that in the meiotic phase there is a true qualitative division brought about by the pairing of the chromosomes during synapsis, and the subsequent separation of whole