The dual force of the dividing cell. Part I: The achromatic spindle figure illustrated by magnetic chains of force

Abstract
Our first description of the polarised figure shown by the cytoplasm of the cell preparing for fission is due to Hermann Fol, who wrote in 1873: “Auf beide Seiten dieser Kernüberbleibsel zeigen sich Plasma-Anhäufungen, deren dicht angesammelten Körnchen zwei regelmässige sternförmige Figuren darstellen. Die Strahle dieser Sterne werden durch die in gerade Linien aneinander gereihten Körnchen gebildet. Mehrere solche Linien reichen von einem Stern oder Anziehungscentrum in einem Bogen zum andern. Das ganze Bild ist äusserst klar, und erinnert lebhaft an die Art und Weise ausgestreuter Eisenstaub sich um die beiden Polen eines Magneten anordnet . . . . Ich schliesse mich ganz und gar der Sachs’schen Theorie der Furchung durch Anziehungs-Mittelpunkte an, nicht etwa aus theoretischen Gründen, sondern weil ich diese Attractionscentren gesehen habe.” Thus the similarity of the cellular field to that of two unlike magnetic poles was recognised from the very outset. The figure in its highest development, as seen in Metazoa (figs. 1—5), has the character of a dumb-bell, whose spheroidal ends are termed “centrosomes,” and with the rays they give off, “polar asters”; while the connection between them, of cytoplasmic fibres, is termed the “spindle.” The astral rays diverge through apparently undifferentiated cytoplasm; but the spindle-fibres traverse or bound a clear space, apparently occupied by liquid during life. During the completion of the figure the nuclear wall has disappeared (fig. 1, E ); of its contents the rod-like bodies, known as chromosomes, are disposed in a symmetrical star across or around the equator of the spindle. The chromosomes now split (fig. 2, G ): the respective sister-segments of all diverge nearly simultaneously, E, and glide to the centrosome (which they may even enter) and then fuse into the daughter nucleus (fig. 2, I, J), the same process taking place simultaneously at either pole. The present study is devoted to the consideration of the character of the force operative in determining the cytoplasmic figure, which we term “mitokinetic force” (our reason for using this term rather than “karyokinetic force” will be found below, p. 564). The behaviour and influence of the chromosomes we pass over for the present.

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