3. Anaphases. — After splitting of the chromosomes, the daughterchromosomes, arranged in two corresponding diverge to opposite poles of the spindle, where they become closely crowded in a mass near the centre of the aster. As they diverge, the two groups of daughter-chromosomes are connected by a bundle of achromatic fibres, stretching across the interval between them, and known as the interzonal fibres or connecting In some cases, these differ in a It was this fact that led Flemming to employ the word "mitosis" (afros, a thread).
2 This stage is termed by Flemming the dyasler, a term which should, however, be abandoned in order to avoid confusion with the earlier word amphiaster. The latter convenient and appropriate term clearly has priority.
3 Verbmdungsfasern of German authors; filaments reunissants of Van Beneden.
marked degree from the other spindle-fibres ; and they are believed by many observers to have an entirely different origin and function. A view now widely held is that of Hermann, who regards these fibres as belonging to a central spindle, surrounded by a peripheral layer of mantle fibres to which the chromosomes are attached, and only exposed to view as the chromosomes separate.' They are sometimes thickened in the equatorial region to form a body known as the cellplate or which, in the case of plant-cells, takes part in the formation of the membrane by which the daughter-cells are separated.
4. Telophases. — In the final phases of mitosis, the entire cell divides in two in a plane passing through the equator of the spindle, each of the daughter-cells receiving a group of chromosomes, half of the spindle, and one of the asters with its centrosome. Meanwhile, a daughter-nucleus is reconstructed in each cell from the group of chromosomes it contains. The nature of this process differs greatly in different kinds of cells. Sometimes, as in the epithelial cells of amphibia, especially studied by Flemming and Rabl, and in many plant-cells, the daughter-chromosomes become thickened, contorted, and closely crowded to form a closely similar to that of the mother-nucleus (Fig. 23); this becomes surrounded by a membrane, the threads give forth branches, and thus produce a reticular nucleus. A somewhat similar set of changes takes place in the segmenting eggs of Ascaris (Van Beneden, Boveri). In other cases, as in many segmenting ova, each chromosome gives rise to a hollow vesicle, after which the vesicles fuse together to produce a single nucleus (Fig. 37). When first formed, the daughter-nuclei are of
equal size. If, however, division of the cell-body has been unequal, the nuclei become, in the end, correspondingly unequal — a fact which, as Conklin and others have pointed out, proves that the size of the nucleus is controlled by that of the cytoplasmic mass in which it lies.
The fate of the achromatic structures varies considerably, and has been accurately determined in only a few cases. As a rule, the spindle-fibres disappear more or less completely, but a portion of their substance sometimes persists in a modified form. In dividing plantcells, the interzonal fibres become thickened at the equator of the spindle and form a transverse plate of granules, known as the cellplate (Fig. 25), which gives rise to the membrane by which the two daughter-cells are separated. The remainder of the spindle disappears. A similar cell-plate occurs in some animal cells; but it is often greatly reduced, and may form only a minute body known as the (Zwischenkorper), which lies between the two cells after their division (Fig. 23). In other cases, as in the cells of the testis, the remains of the spindle in each cell sometimes gives rise to a more or less definite body known as the paranucleus or Nebenkern (Fig. 62).
The aster may in some cases entirely disappear, together with the centrosome (as occurs in the mature egg). In a large number of cases, however, the centrosome persists, lying either outside or more rarely inside the nucleus and dividing into two at a very early period. This division is clearly a precocious preparation for the ensuing division of the daughter-cell, and it is a remarkable fact that it occurs as a rule during the early anaphase, before the mother-cell itself has divided. There are, however, some undoubted cases (cf: Figs. 6, 7) in which the centrosome remains undivided during the resting stage and only divides as the process of mitosis begins.
Like the centrosome, the aster or its central portion may persist in a more or less modified form throughout the resting state of the cell, forming a structure generally known as the attraction-sphere. This body often shows a true astral structure with radiating fibres (Figs. 7, 35); but it is sometimes reduced to a regular spherical mass which may represent only the centrosphere of the original aster (Fig. 6).