to consider the cell in relation to yet another vital attribute, that of reproduction. Just as we now know that the phenomena of assimilation, respiration, excretion, response, movement and so forth, characteristic of living things, are but the co-ordinated expressions of the corresponding activities of the constituent cells, so we now know that the reproduction of the organism is, in its ultimate analysis, a cell-process. Our knowledge of the essential fact that cells only arise by the division of pre-existing cells, now a fundamental axiom of biology, and of the details of this process, have been acquired during recent years by the strenuous efforts of numerous workers.[1] Matthias Jakob Schleiden (1838) supposed that in plants the new cell arose from the parent cell by a sort of “crystallizing” process from the cell fluid or “cytoblastema”; the nucleolus appearing first, then the nucleus, and finally the cell-body. Theodor Schwann (1839) extended Schleiden’s theory to animal tissues, with this yet greater error, that new cells might arise, not only within the mother cell as Schleiden had supposed, but also in the intercellular substance so common in animal tissues (to which he also gave the term “cytoblastema”). By 1846, however, the botanists, thanks mainly to the efforts of Hugo von Mohl and Nägeli, recognized as a general law that cells only arise by the division of a pre-existing cell. But it was long before the universal application of this law was recognized by zoologists; the delay being largely due to pathological phenomena. The work of Kölliker (1844–1845), Karl Bogislaus Reichert (1841–1847), and Remak (1852–1855), however, finally enabled Virchow in 1858 to maintain the law of the genetic continuity of cells in the since famous aphorism omnis cellula e cellula. At this time, however, nothing was known of the details of cell-division,—one school (Reichert, L. Auerbach, and the majority of the botanists) maintaining that the nucleus disappeared prior to cell-division, the other school (von Baer, Remak, Leydig, Haeckel, &c.) maintaining that it took a leading part in the process. It is not until the appearance of Anton Schneider’s work in 1873, followed by those of Fol, Auerbach, Strasburger and many others, that we begin to gain an insight into the process. In 1882 W. Flemming was able to extend Virchow’s aphorism to the nucleus also: omnis nucleus e nucleo.
Outline of Cell-division.—There are two very distinct methods of cell-division. The more general and also more complicated method is accompanied by the formation of a complex fibrillar mechanism, and was on this account termed “mitosis” (μίτος, a thread) by W. Flemming (1882), and “karyokinesis” (κάρυον, nut, nucleus, and κίνησις, change, movement) by W. Schleicher (1878). The other method, “amitosis,” or direct division, is unaccompanied by any visible mechanism and is of relatively exceptional occurrence. In the more usual method of cell-division, or “mitosis,” we can distinguish two distinct but parallel processes, the one undergone by the chromatin and resulting in the “chromatic figure,” the other usually only concerning the cytoplasm and resulting in the “achromatic figure.”[2]
a, b and c from Prof. E. B. Wilson’s The Cell in Development and Inheritance, by permission of the author and the Macmillan Co., New York; d from A. Gurwitsch, Morphologie u. Biologie der Zelle, by permission of Gustav Fischer
Fig. 6.—Diagram of Nuclear Division. a, Spireme stage; b, Spindle formed; c, Spindle complete; equatorial plate formed; d, Division completed.
We will consider the chromatin changes first. The chromatin granules lose their scattered arrangement on the nuclear reticulum, and become instead arranged in a linear series to form a coiled and deeply staining “spireme thread”[3] (fig. 6, a). As the thread contracts, its granular origin becomes less evident, and at the same time the coils become fewer in number; the “close” spireme of earlier stages becomes the “loose” spireme of later stages. As the spireme thread contracts, it segments into a number of short, and usually U-shaped, segments—the “chromosomes” (Waldeyer, 1888). The number of these chromosomes is always constant for the cells of any given species of plant or animal, but varies greatly in number in different species. Thus in the parasitic worm Ascaris megalocephala, var. univalens, there are only two. In the crustacean Artemia Bauer found 168, while in the amphibian Salamandra maculata, as also in the lily, the number is 24. While these changes have been proceeding in the nucleus, changes in the cytoplasm have resulted in the formation of the achromatic figure. These cytoplasmic changes are initiated by the division into two of a minute body, the “centrosome,” originally discovered by P. J. van Beneden in 1883,[4] and usually lying not far from the nucleus (fig. 6, a). The daughter centrosomes separate from one another, travelling to opposite poles of the nucleus. At the same time radiations extend out into the cytoplasm from the centrosomes, and, as the nuclear membrane disappears, invade the nuclear area (fig. 7, a). Some of the fibrillae in the latter region become attached to the chromosomes and are termed “mantle fibres”; others become continuous from one centrosome to the other and constitute the “spindle fibres.” The remaining radiations at the two poles of the spindle are the “astral rays.” (The details of the formation of the achromatic figure vary considerably, some indication of this is given in the next section in connexion with the question of the origin of the mitotic mechanism.) The chromosomes now arrange themselves in the “equatorial plate” of the spindle and each splits longitudinally into two[5] (fig. 6, b and c). The sister chromosomes now pass to opposite poles of the spindle (fig. 6, d), and there, returning to the “resting” condition, constitute the daughter nuclei. Division of the cell follows, usually, in animals, by simple constriction. Both Theodor Boveri and van Beneden, in their papers of 1887, regarded the centrosome as initiating, not only the division of the cell-body but that of the chromatin also; Beneden even suggested that the pull of the mantle fibres caused the division of the chromatin in the equatorial plate. W. Pfitzner in 1882 was the first to show that the splitting of the chromosomes in the equatorial plate was only the reappearance of a split in the spireme thread and was due to a corresponding
- ↑ Prominent among these are: Schleiden (1873), Fol (1873–1877), Auerbach (1874), Bütschli (1876), Strasburger (1875–1888), O. Hertwig (1875–1890), R. Hertwig (1875–1877); Flemming (1879–1891), van Beneden (1883–1887), Rabl (1889), Boveri (1887–1903).
- ↑ This distinction between the chromatic and achromatic portions of the mitotic figure is due to Flemming.
- ↑ The genesis of the spireme thread was first described by E. G. Balbiani in 1876.
- ↑ “Recherches sur la maturation de l’œuf, la fécondation et la division cellulaire” (Archives de biologie, vol. iv.).
- ↑ First discovered by Flemming in 1879 and confirmed by Retzius in 1881.
