That this chromatin must be something quite special we see from the processes of cell and nuclear division, which I shall now briefly describe.
Fig. 74. Diagram of nuclear division, adapted from E. B. Wilson. A, resting cell with cell-substance (zk), centrosphere (csph) which contains two centrosomes, nucleolus (kk); and chromosomes (chr), the last distributed in the nuclear reticulum. B, the chromatin united in a coiled thread; the centrosphere divided into two and giving off rays which unite the halves. C, the nuclear spindle (ksp) formed, the rays more strongly developed, the nuclear membrane (km) in process of dissolution, the chromatin thread divided into eight similar pieces (chrs), the rays are attaching themselves to the chromosomes. D, perfected nuclear spindle with the two centrospheres at the poles (csph) and the eight chromosomes (chrs) in the equator of the spindle, all now longitudinally split. E, daughter-chromosomes diverging from one another, but still united by filaments, the centrosomes (cs) are already doubled for the next division. F, daughter-chromosomes, quite separated from one another, are already beginning to give off processes; the cell-substance is beginning to be constricted. G, end of the process of division: two daughter-cells (tz) with similar nuclear reticulum (tk) and centrospheres (csph), as in A.
When a cell is on the eve of dividing we observe first that the chromatin grains, which have till then been scattered throughout the network of the nucleus, approach each other and arrange themselves into a long thin thread which, irregularly intertwined, forms a loose skein, the so-called coil-stage (Fig. 74, B). The thread then begins to thicken, and somewhat later it can be seen to have broken up into a number of pieces of equal length, as if it had been cut into equal pieces with scissors (C).
These pieces or chromosomes become shorter by slowly contracting, and thus each takes the form of an angular loop, a straight rod, or a roundish, oval, or spherical body (Fig. 74, C, chrs). While this is happening, we can see at the side of the nucleus, and closely apposed to it, a pale longitudinally striped figure with a swelling, similar to a handle, at both ends—the so-called nuclear spindle or central spindle (ksp). This is the apparatus for the division of the nucleus, and it was previously represented by a small body susceptible to certain stains—the centrosome, which was surrounded by a halo-like zone, the centrosphere or 'sphere.' This body was long overlooked, but now the majority of investigators assume that, though it is often inconspicuous and very difficult to make visible, it is nevertheless present in every cell which is capable of division, and that it is therefore a permanent and indispensable constituent of the cell (Fig. 74, A and B, csph).
When a cell is on the point of dividing, this remarkable cell-organ, which has hitherto seemed no more than an insignificant, pale, little sphere, now becomes active. First of all, often before the formation of the chromatin coil, it doubles by division ([A and B csph]), at first only as regards the centrosome, and then as regards the sphere also ([B]); and while division is going on fine protoplasmic filaments issue from the dividing sphere and radiate like rays from a sun into the cell-substance. As they only retain their connexion with each other at the surfaces of the dividing halves of the sphere which are turned towards each other, we might almost say that fine threads are drawn out between the two halves as they separate, and these become longer the further apart the halves diverge. In this manner the much-talked-of 'spindle figure' arises, which was first described in the seventies through the researches of A. Schneider, Auerbach, and Bütschli, but the significance and origin of which have claimed the labours of many later investigators down to our own day.
The processes now to be described do not always take place in exactly the same manner, but the gist of the business is everywhere the same, and it consists in this, that the two ends or 'poles' of the spindle diverge further and further apart, and between them lies the nucleus whose membrane now disappears ([C, km]) while the spindle threads traverse its interior. Sometimes the membrane is retained, but nevertheless the spindle threads penetrate into the interior of the nucleus. But the chromosomes always range themselves quite regularly in the 'equatorial plane' of the spindle ([D, aeq])—a process the precise mechanism of which is by no means clearly understood, and indeed the play of the forces in the whole process of nuclear division is still very imperfectly revealed to our intelligence.
Thus we have now before us a pale, spindle-shaped figure, which takes only a faint stain, with the 'suns' ([cs]) at its 'poles,' and in its equatorial plane the loop- or rod-shaped, or spherical chromosomes ([chrs]). The whole is designated the 'karyokinetic,' the 'mitotic,' or the 'nuclear division figure.'
The meaning and importance of this, at first sight, puzzling figure will at once become clear from what follows. It may be observed at this stage, if not even long before, that each of the chromatin rods or loops has split along its whole length like a log of wood, and that the split halves are beginning slowly and hardly noticeably to move away from each other, one half towards one, the other towards the other pole of the spindle ([Fig. D and F]). Directly in front of the centrosome they make a halt, and now the material for the two daughter-nuclei is in its proper place ([F, chrs]). These develop quickly, each chromosome group surrounding itself with a nuclear membrane ([Fig. G]) within which the chromosomes gradually become transformed again into a nuclear network. Within the chromatin substance proper this is scattered about in small roundish or angular granules, lying especially at the intersecting points of the network. It may be stated at once, though the full significance of the statement can only be appreciated later, that we may assume with probability that this breaking up of the chromosomes is only apparent, and that these rods or spheres really continue to exist in the nuclear network, only in a different form, greatly spread out, somewhat after the manner of a Rhizopod which stretches out fine processes in all directions. These processes branch and anastomose, so that the body, which previously seemed compact, now appears as a fine network. In point of fact, it can be directly observed that the chromosomes, after the nucleus is completely divided into two daughter-nuclei, send out pointed processes ([F and G]) which gradually increase in length and branch, while the body of the chromosome itself becomes gradually smaller. It is thus probable that, when such a daughter-nucleus is on the point of dividing anew, it may, by a drawing together of the processes or pseudopodia of the chromosomes, produce the same rods or spheres as those which previously gave rise to the network. More definite reasons for this interpretation will be adduced later on. In any case, the chromosomes, even in their compact rod-like state, consist of two kinds of substance, the chromatin proper, which stains deeply, and the linin, which is difficult to stain; and it is the latter which, by breaking up, forms the pale part of the nuclear network.