The visible field of force, though often called the “nuclear spindle,” is formed outside of, but usually near to, the nucleus. Let us look a little more closely into the structure of this body, and into the changes which it presently undergoes.
Within its spherical outline (Fig. [42]), it contains an “alveolar” {171} meshwork (often described, from its appearance in optical section, as a “reticulum”), consisting of more solid substances, with more fluid matter filling up the interalveolar meshes. This phenomenon is nothing else than what we call in ordinary language, a “froth” or a “foam.” It is a surface-tension phenomenon, due to the interacting surface-tensions of two intermixed fluids, not very different in density, as they strive to separate. Of precisely the same kind (as Bütschli was the first to shew) are the minute alveolar networks which are to be discerned in the cytoplasm of the cell[227], and which we now know to be not inherent in the nature of protoplasm, or of living matter in general, but to be due to various causes, natural as well as artificial. The microscopic honeycomb structure of cast metal under various conditions of cooling, even on a grand scale the columnar structure of basaltic rock, is an example of the same surface-tension phenomenon. {172}
| Fig. 42. | Fig. 43. |
But here we touch the brink of a subject so important that we must not pass it by without a word, and yet so contentious that we must not enter into its details. The question involved is simply whether the great mass of recorded observations and accepted beliefs with regard to the visible structure of protoplasm and of the cell constitute a fair picture of the actual living cell, or be based on appearances which are incident to death itself and to the artificial treatment which the microscopist is accustomed to apply. The great bulk of histological work is done by methods which involve the sudden killing of the cell or organism by strong reagents, the assumption being that death is so rapid that the visible phenomena exhibited during life are retained or “fixed” in our preparations. While this assumption is reasonable and justified as regards the general outward form of small organisms or of individual cells, enough has been done of late years to shew that the case is totally different in the case of the minute internal networks, granules, etc., which represent the alleged structure of protoplasm. For, as Hardy puts it, “It is notorious that the various fixing reagents are coagulants of organic colloids, and that they produce precipitates which have a certain figure or structure, ... and that the figure varies, other things being equal, according to the reagent used.” So it comes to pass that some writers[228] have altogether denied the existence in the living cell-protoplasm of a network or alveolar “foam”; others[229] have cast doubts on the main tenets of recent histology regarding nuclear structure; and Hardy, discussing the structure of certain gland-cells, declares that “there is no evidence that the structure discoverable in the cell-substance of these cells after fixation has any counterpart in the cell when living.” “A large part of it” he goes on to say “is an artefact. The profound difference in the minute structure of a secretory cell of a mucous gland according to the reagent which is used to fix it would, it seems to me, almost suffice to establish this statement in the absence of other evidence.”
Nevertheless, histological study proceeds, especially on the part of the morphologists, with but little change in theory or in method, in spite of these and many other warnings. That certain visible structures, nucleus, vacuoles, “attraction-spheres” or centrosomes, etc., are actually present in the living cell, we know for certain; and to this class belong the great majority of structures (including the nuclear “spindle” itself) with which we are at present concerned. That many other alleged structures are artificial has also been placed beyond a doubt; but where to draw the dividing line we often do not know[230]. {173}
The following is a brief epitome of the visible changes undergone by a typical cell, leading up to the act of segmentation, and constituting the phenomenon of mitosis or caryokinetic division. In the egg of a sea-urchin, we see with almost diagrammatic completeness what is set forth here[231].
| Fig. 44. | Fig. 45. |
- 1. The chromatin, which to begin with was distributed in granules on the otherwise achromatic reticulum (Fig. [42]), concentrates to form a skein or spireme, which may be a continuous thread from the first (Figs. [43], 44), or from the first segmented. In any case it divides transversely sooner or later into a number of chromosomes (Fig. [45]), which as a rule have the shape of little rods, straight or curved, often bent into a V, but which may also be ovoid, or round, or even annular. Certain deeply staining masses, the nucleoli, which may be present in the resting nucleus, do not take part in the process of chromosome formation; they are either cast out of the nucleus and are dissolved in the cytoplasm, or fade away in situ.
- 2. Meanwhile, the deeply staining granule (here extra-nuclear), known as the centrosome, has divided in two. The two resulting granules travel to opposite poles of the nucleus, and {174} there each becomes surrounded by a system of radiating lines, the asters; immediately around the centrosome is a clear space, the centrosphere (Figs. [43]–45). Between the two centrosomes with their asters stretches a bundle of achromatic fibres, the spindle.
- 3. The surface-film bounding the nucleus has broken down, the definite nuclear boundaries are lost, and the spindle now stretches through the nuclear material, in which lie the chromosomes (Figs. [45], 46). These chromosomes now arrange themselves midway between the poles of the spindle, where they form what is called the equatorial plate (Fig. [47]).
- Fig. 46.
- Fig. 47.
- 4. Each chromosome splits longitudinally into two: usually at this stage,—but it is to be noticed that the splitting may have taken place so early as the spireme stage (Fig. [48]).
- 5. The halves of the split chromosomes now separate from one another, and travel in opposite directions towards the two poles (Fig. [49]). As they move, it becomes apparent that the spindle consists of a median bundle of “fibres,” the central spindle, running from pole to pole, and a more superficial sheath of “mantle-fibres,” to which the chromosomes seem to be attached, and by which they seem to be drawn towards the asters.
- 6. The daughter chromosomes, arranged now in two groups, become closely crowded in a mass near the centre of each aster {175} (Fig. [50]). They fuse together and form once more an alveolar reticulum and may occasionally at this stage form another spireme.
- Fig. 48.
- Fig. 49.
- A boundary or surface wall is now developed round each reconstructed nuclear mass, and the spindle-fibres disappear (Fig. [51]). The centrosome remains, as a rule, outside the nucleus.
- Fig. 50.
- Fig. 51.
- 7. On the central spindle, in the position of the equatorial plate, there has appeared during the migration of the chromosomes, a “cell-plate” of deeply staining thickenings (Figs. [50], 51). This is more conspicuous in plant-cells. {176}
- 8. A constriction has meanwhile appeared in the cytoplasm, and the cell divides through the equatorial plane. In plant-cells the line of this division is foreshadowed by the “cell-plate,” which extends from the spindle across the entire cell, and splits into two layers, between which appears the membrane by which the daughter cells are cleft asunder. In animal cells the cell-plate does not attain such dimensions, and no cell-wall is formed.