The Mechanism of Mitosis

the shortening and thickening of these rays as the daughter-chromosomes diverge. In all these operations the behaviour of the rays is precisely like that of muscle-fibres ; and it is difficult to study Boveri's beautiful figures and clear descriptions without sharing his conviction that " of the contractility of the fibrillm there can be no Very convincing evidence in the same direction is afforded by pigment-cells and leucocytes or wandering-cells, in both of which there is a very large permanent aster (attraction-sphere) even in the resting-cell. The structure of the aster in the leucocyte, where it was first discovered by Flemming in 1891, has been studied very carefully by Heidenhain in the salamander. The astral rays here extend throughout nearly the whole cell (Fig. 35), and are believed by Heidenhain to represent the contractile elements by means of which the cell changes its form and creeps about. A similar conclusion was reached by Solger 090 and Zimmerman ('93, 2) in the case of pigment-cells (chromatophores) in fishes. These cells have, in an extraordinary degree, the power of changing their form, and of A. Entire pigment-cell, from Bleresisu. The central clear space is the central mass of the aster from which radiate the pigment-granules ; two nuclei below. B. Nucleus (n) and aster after extraction of the pigment, showing reticulated central mass. C. Two nuclei and aster with rodshaped central mass, from Sargus.

actively creeping about. Solger and Zimmerman have shown that the pigment-cell contains an enormous aster, whose rays extend in every direction through the pigment-mass, and it is almost impossible to doubt that the aster is a contractile apparatus, like a radial muscular system, by means of which the active changes of form are produced (Fig. 36).

But although these observations seem to place the theory of fibrillar contractility upon a firm basis, it has since undergone various modifications and limitations, which show that the matter is by no means so simple as it first appeared. The most important of these modifications are due to Hermann ('91) and Droner ('95), who have relied mainly on the study of mitosis in various cells of the salamander, well known as extremely favourable objects for study. These observers have demonstrated that in this case the spindle-fibres are of two kinds which, apparently, differ both in origin and in mode of action. Hermann showed that the primary amphiaster is formed outside the nucleus, without connection with the chromosomes, and that the original spindle persists as a " central spindle " (Figs. 21, 22), which he regards as composed of non-contractile fibres, and merely forming a support on which the movements of the chromosomes take place. The contractile elements are formed by certain of the astral rays which grow into the nucleus, and become attached to the chromosomes, as Boveri described. By the contraction of these latter fibres the chromosomes are now dragged towards the spindle, and around its equator they are finally grouped to form the equatorial plate. The

fully formed spindle consists, therefore, of two elements ; namely, (a) the original " central spindle," and (b) a surrounding mantle of contractile " mantle-fibres " attached to the chromosomes, and originally derived from astral rays. In the anaphase, as Hermann believes, the daughter-chromosomes are dragged apart solely by the contractile mantlefibres, the central being non-contractile and serving as a support or substratum along which the chromosomes move. As the chromosomes diverge, the central spindle comes into view as the interzonal fibres (Fig. 22, G, H). Strasburger ('95) is now inclined to accept a similar view of mitosis in the cells of plantq.

Droner ('95) in his beautiful studies on the mechanism of mitosis has advanced a step beyond Hermann, maintaining that the progressive divergence of the spindle-poles is caused by an active growth or elongation of the central spindle which goes on throughout the whole period from the earliest prophases until the close of the anaphases. This view is supported by the fact that the central spindle-fibres are always contorted during the metaphases, as if pushing against a resistance ; and, as Richard Hertwig points out ('95), it harmonizes with the facts observed in the mitoses of infusorian nuclei. The same view is adopted by Braus and by Reinke. Flemming ('95) is still inclined, however, to the view that the divergence of the centres may be in part caused by the traction of the antipodal fibres, as maintained by Van Beneden and Boveri.

Heidenhain, finally, while accepting the contractility-hypothesis, ascribes only a subordinate role to an active physiological contractility of the fibres. The main factor in mitosis is ascribed to elastic tension of the astral rays which are attached at one end to the centrosome, at the other to the cell-periphery. By turgor of the cell the rays are passively stretched, thus causing divergence of the spindle-poles and of the daughter-chromosomes to which the spindle-fibres are attached. An active contraction of the fibres is only invoked to explain the closing phases of mitosis.

(b) Other Theories. — Watase's ingenious theory of mitosis ('93) is exactly the opposite of Van Beneden's, assuming that the spindlefibres are not pulling but pushing agents, the daughter-chromosomes being forced apart by continually lengthening fibres which grow out from the centres and dovetail in the region of the interzonal fibres. Each daughter-chromosome is therefore connected with fibres from the aster, not of its own, but of the opposite side. This view is, I believe, irreconcilable with the movements of chromosomes observed in multiple asters, and also with those that occur during the fertilization of the egg, where the chromosomes are plainly drawn towards the astral centres and not pushed away from them.