CYTOLOGY figure—Aster—when seen from the pole of the spindle. This is called the nuclear plate (Fig. 1, 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. 1, 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.— they reach the poles the chromosomes group themselves again in the form of stars—Diaster—with spindle-fibres extending between them (Fig. 1, K). The chromosomes then fuse together again to form a single thread (Fig. 1, 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. When this nuclear division is followed by cell-division, a cell-wall is formed between the two daughter nuclei. The spindle-fibres which extend between the daughter nuclei increase in number and extend laterally until they touch the cell-walls. They then thicken in the middle, and form a layer of granules across the cell—the cell-plate. These then disappear and the new cell-wall appears in their place. In the lower plants the new cell-wall may be formed as an outgrowth on the old one (Spirogyra, &c.), or it may be formed directly in the cytoplasm. 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. It 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 longitudinal splitting of the chromosomes appears to take place in order to ensure that one-half of each chromatin unit—id—shall be distributed to each daughter nucleus. The longitudinal division of the linin thread is in fact probably initiated by the division of these chromatin granules, which are arranged in a single row along the thread. In many cases the granular nature of the chromosome is masked by the shortening of the thread and the absorption of additional chromatin substances during the various stages of the prophase, so that the chromosome appears to be a homogeneous structure. In other cases the longitudinal splitting takes place before the chromosome stage, in which case two rows of granules are visible on the linin thread at an early stage, and the shortening and thickening do not take place until the daughter chromosomes have become visible as separate threads. This phenomenon is especially prominent in the nuclear divisions which lead to the formation of the sexual cells. Here certain other features also present themselves which are not observed in the vegetative nuclei, the most important being the reduction in the number of chromosomes—to one-half. This takes place in the spore mothercells of all those Vascular Cryptogams and Phanero-
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gams which have been investigated, and is probably of general occurrence. We know very little of the phenomenon as it takes place in the lower plants, but in Incus it occurs in the oogonial stage, so that the cells of the vegetative plant have the full number. Inasmuch as the essential feature of fertilization is the fusion of two nuclei, it is evident that unless this reduction took place the number of chromosomes would be doubled at each generation. In the higher plants this reduced numbei is found in all the cells of the gametophyte, the full
Fig. i.—Various stages in the nuclear division of the pollen mother-cells of Lilium. {After Gnigoire.) number in those of the sporophyte. The reduction is brought about simply by the segmentation of the spirem thread into half the number of segments instead of the normal number; but there is no diminution in the actual amount of nuclear substance. During the second division, however, it has been stated that a qualitative reduction is brought about by the transverse division of the chromosomes, and that different chromatin granules, or ids, are thus distributed to the daughter nuclei.. This has an important bearing upon the supposed function of the ids as the bearers of hereditary qualities in fertilization; but the most recent investigations seem to show that, so far as
