When the lines of force stream inwards from the periphery towards a point in the interior of the cell, the particles susceptible of attraction either crowd towards the surface of the cell, or, when retarded by friction, are seen forming lines or “fibrillae” which radiate outwards from the centre and constitute a so-called “aster.” In the cells of columnar or ciliated epithelium, where the sides of the cell are symmetrically disposed to their neighbours but the free and attached surfaces are very diverse from one another in their external relations, it is these latter surfaces which constitute the opposite poles; and in accordance with the parallel lines of force so set up, we very frequently see parallel lines of granules which have ranged themselves perpendicularly to the free surface of the cell (cf. fig. [97]).
A simple manifestation of “polarity” may be well illustrated by the phenomenon of diffusion, where we may conceive, and may automatically reproduce, a “field of force,” with its poles and visible lines of equipotential, very much as in Faraday’s conception of the field of force of a magnetic system. Thus, in one of Leduc’s experiments[224], if we spread a layer of salt solution over a level {168} plate of glass, and let fall into the middle of it a drop of indian ink, or of blood, we shall find the coloured particles travelling outwards from the central “pole of concentration” along the lines of diffusive force, and so mapping out for us a “monopolar field” of diffusion: and if we set two such drops side by side, their lines of diffusion will oppose, and repel, one another. Or, instead of the uniform layer of salt solution, we may place at a little distance from one another a grain of salt and a drop of blood, representing two opposite poles: and so obtain a picture of a “bipolar field” of diffusion. In either case, we obtain results closely analogous to the “morphological,” but really dynamical, polarity of the organic cell. But in all probability, the dynamical polarity, or asymmetry of the cell is a very complicated phenomenon: for the obvious reason that, in any system, one asymmetry will tend to beget another. A chemical asymmetry will induce an inequality of surface-tension, which will lead directly to a modification of form; the chemical asymmetry may in turn be due to a process of electrolysis in a polarised electrical field; and again the chemical heterogeneity may be intensified into a chemical “polarity,” by the tendency of certain substances to seek a locus of greater or less surface-energy. We need not attempt to grapple with a subject so complicated, and leading to so many problems which lie beyond the sphere of interest of the morphologist. But yet the morphologist, in his study of the cell, cannot quite evade these important issues; and we shall return to them again when we have dealt somewhat with the form of the cell, and have taken account of some of the simpler phenomena of surface-tension.
We are now ready, and in some measure prepared, to study the numerous and complex phenomena which usually accompany the division of the cell, for instance of the fertilised egg.
Division of the cell is essentially accompanied, and preceded, by a change from radial or monopolar to a definitely bipolar polarity.
In the hitherto quiescent, or apparently quiescent cell, we perceive certain movements, which correspond precisely to what must accompany and result from a “polarisation” of forces within the {169} cell: of forces which, whatever may be their specific nature, at least are capable of polarisation, and of producing consequent attraction or repulsion between charged particles of matter. The opposing forces which were distributed in equilibrium throughout the substance of the cell become focussed at two “centrosomes,” which may or may not be already distinguished as visible portions of matter; in the egg, one of these is always near to, and the other remote from, the “animal pole” of the egg, which pole is visibly as well as chemically different from the other, and is the region in which the more rapid and conspicuous developmental changes will presently begin. Between the two centrosomes, a spindle-shaped
Fig. 41. Caryokinetic figure in a dividing cell (or blastomere) of the Trout’s egg. (After Prenant, from a preparation by Prof. P. Bouin.)
figure appears, whose striking resemblance to the lines of force made visible by iron-filings between the poles of a magnet, was at once recognised by Hermann Fol, when in 1873 he witnessed for the first time the phenomenon in question. On the farther side of the centrosomes are seen star-like figures, or “asters,” in which we can without difficulty recognise the broken lines of force which run externally to those stronger lines which lie nearer to the polar axis and which constitute the “spindle.” The lines of force are rendered visible or “material,” just as in the experiment of the iron-filings, by the fact that, in the heterogeneous substance of the cell, certain portions of matter are more “permeable” to the acting force than the rest, become themselves polarised after the {170} fashion of a magnetic or “paramagnetic” body, arrange themselves in an orderly way between the two poles of the field of force, cling to one another as it were in threads[225], and are only prevented by the friction of the surrounding medium from approaching and congregating around the adjacent poles.
As the field of force strengthens, the more will the lines of force be drawn in towards the interpolar axis, and the less evident will be those remoter lines which constitute the terminal, or extrapolar, asters: a clear space, free from materialised lines of force, may thus tend to be set up on either side of the spindle, the so-called “Bütschli space” of the histologists[226]. On the other hand, the lines of force constituting the spindle will be less concentrated if they find a path of less resistance at the periphery of the cell: as happens, in our experiment of the iron-filings, when we encircle the field of force with an iron ring. On this principle, the differences observed between cells in which the spindle is well developed and the asters small, and others in which the spindle is weak and the asters enormously developed, can be easily explained by variations in the potential of the field, the large, conspicuous asters being probably correlated with a marked permeability of the surface of the cell.