Nucleolus

“ The division of the nucleolus, so marked in the plant, has not been observed in the animal, and may be, in the latter, suppressed or hastened, like many of the developmental changes in the higher animals. On the other hand, the peculiar movements of the chromatin-fibrils of the animal cell have no direct counterpart in the plant. They seem to present a distinct step forward in cellular evolution, and yield the idea that the animal cell is a more advanced organism than that of the vegetable. ”

“ If this be abandoned, then the first work upon the chromatin structure of Pentatoma is practically discredited, for Montgomery has definitely retreated from his positions concerning the absence of the “chromatin nucleolus” in the spermatogonia, the non-occurrence of a longitudinal cleft in the spireme thread, the lack of an equational division of the chromatin in the spermatocyte, the origin of the “chromatin nucleolus,” and the fragmentation of the “chromatin nucleolus.” ”

“ The researches of the last twenty years have shown that the structure of the nucleus and the phenomena of nuclear division in these lower forms conforms in all essential details to those in the higher plants. Thus in the Basidiomycetes (fig. 7) the nuclei possess all the structures found in the higher plants, nuclear membrane, chromatin network and nucleolus (fig. 7, B) , and in the process of division, chromosomes, nuclear spindle and centrosomes are to be seen (fig. 7, C-G) . ”

“ For a stronger reason, we cannot imagine that any of the atoms of which any cell is composed, has these faculties. Nor can we conceive that a portion of these atoms, automatically, metamorphose themselves into a nucleolus, others into a nucleolinus, while the remainder continue to be a simple cell-body. ”

“ If the spore mother cells are still quite young, we shall find the nucleus (Fig. 127, A, n) comparatively small, and presenting a granular appearance when strongly magnified. These granules, which appear isolated, are really parts of filaments or segments, which are closely twisted together, but scarcely visible in the resting nucleus. On one side of the nucleus may usually be seen a large nucleolus (called here, from its lateral position, paranucleus) , and the whole nucleus is sharply separated from the surrounding protoplasm by a thin but evident membrane. ”

“ Originally solid, the nucleus is commonly differentiated later into an outer dense nuclear-membrane and an inner softer or fluid content; either with one single nucleolus or with a variable number of nucleoli. Originally always simple, the nucleus becomes afterwards constantly divided into numerous small nuclei, each of which, together with a part of the surrounding protoplasm, forms a vibratile-spore or "flagellate-spore." This division in the Acantharia and in the social (or colonial) Peripylea begins very early, in all other Radiolaria much later, immediately before propagation. ”

“ Spicula long and thin, cylindrical, smooth, more or less bent, pointed at both ends, similar to those of Thalassoplancta cavispicula. Central capsule thin-walled, without oil-globules, four times as broad as the nucleus, which encloses one single nucleolus. ”

“ In anucleate cells there is probably only a single process of layer formation round an infinitely small nucleolus. In almost all nucleate cells the nucleus is resorbed when the cell reaches its full development, and it is larger and more important the younger the cell is. ”

“ There is no special nucleolus. The rapid growth of the germinal epithelium is due [Pg 596] to the division of its cells, and great masses of these may frequently be seen to be undergoing division at the same time. Of the tissue of the ovary internal to the germinal epithelium, it may be noticed that the tubuliferous tissue derived from the Malpighian bodies is no longer in contact with the germinal epithelium, but that a layer of vascular stroma is to a great extent interposed between the two. The vascular stroma of the hilus has, moreover, greatly increased in quantity. ”

“ In cell-division the first stage is the aggregation, on opposite sides of the nucleus, in a line with the long axis of the cell, of a quantity of pale, slightly granular protoplasm. This is perhaps derived from the peripheral layer, and travels inward along the fibers, since minute thickenings, of similar appearance, show themselves upon these fibers. The nucleolus swells out into two protuberances, in the same axial line, joined by a bridge of denser matter. ”

“ The nucleus, which in many cells is the largest and most central body, is of special importance. It contains a number of threadlike bodies, or chromosomes, that are the carriers of the cell’s heredity-controlling system. These contain granules of a material called chromatin, which is rich in a nucleic acid, DNA (deoxyribonucleic acid) . The chromosomes usually are not readily seen in the nucleus except when the cell, along with its nucleus, is dividing. When the nucleus is not dividing, a spherical body, the nucleolus, can be seen. (In some nuclei there may be more than one.) ”

“ This body is called the nucleus of the cell, and the small one within it, the nucleolus.
The membrane surrounding the cell is known as the cell wall, and in young plant cells is always composed of cellulose.
The granular substance lining the cell wall (Fig. 1, pr.) is called “protoplasm,” and with the nucleus constitutes the living part of the cell. If sufficiently magnified, the granules within the protoplasm will be seen to be in active streaming motion. This movement, which is very evident here, is not often so conspicuous, but still may often be detected without difficulty. ”

“ The most important feature to be noted in this connection is the fact that the structure does not exist as a simple element, but is observed as a number of granules, and that this number varies considerably in different species. These granules fuse during the “synapsis stage,” as do the chromosomes, to produce in the spermatocyte half the number of “chromatin nucleoli” that were present in the spermatogonia. In this respect the “chromatin nucleolus” differs radically from the accessory chromosome, which has the same valence in both cell generations. ”

“ The mode of origin of the nucleus and the part which it plays in the production of new cells have been the subject of much discussion. Schleiden, whose observations, published in 1838, were made on the cells of plants, believed that within the cell a nucleolus first appeared, and that around it molecules aggregated to form the nucleus. Schwann again, whose observations were mostly made on the cells of animals, considered that an amorphous material existed in organized bodies, which he called cytoblastema. ”

“ In the spermatogonium there is a very large nucleolus (plate I, fig. 1) , which in the iron-hæmatoxylin preparations is very conspicuous, but does not stain like chromatin with thionin or other anilin stains, nor does it behave like an accessory chromosome during the maturation mitoses. Before each spermatogonial division it divides as in figures 2 and 3, and the same is true for each maturation mitosis. Figure 4 shows the 52 chromosomes of a spermatogonial division in metaphase. Figures 5 and 6 are young spermatocytes, showing the division of the nucleolus. ”

“ The former, for instance, contains a rounded nucleolus or “germinal spot,” certain conspicuous granules or strands of the peculiar substance called chromatin, and a coarse meshwork of a protoplasmic material known as “linin” or achromatin; the outer protoplasm, or cytoplasm, is generally believed to consist throughout of a sponge-work, or rather alveolar meshwork, of more and less fluid substances; and lastly, there are generally to be detected one or more very minute bodies, usually in the cytoplasm, sometimes within the nucleus, known as the centrosome or centrosomes. ”

“ The nucleus represents the most vital part of the cell protoplasm, and has a more granular appearance than the latter. The nucleolus is a minute solid spot in a nucleus, appearing to be more highly refractive. ”

“ For microscopical examination the larger species are preferable. When one of these is magnified (Fig. 18, A, C) , the unbranched filament is shown to be made up of perfectly cylindrical cells, with rather delicate walls. The protoplasm is confined to a thin layer lining the walls, except for numerous fine filaments that radiate from the centrally placed nucleus (n) , which thus appears suspended in the middle of the cell. The nucleus is large and distinct in the larger species, and has a noticeably large and conspicuous nucleolus. ”

“ The cells in the inner mass are classified into (1) undifferentiated cells, recognised by their nucleus, which possesses a nucleolus; these are the archaeocytes; (2) differentiated cells, of which the nucleus contains a chromatin net; these give rise to pinacocytes, collencytes, and scleroblasts. Some of them form a flat epithelium, which covers the hinder pole. Some of the scleroblasts already contain spicules. Fixation occurs very early. ”

“ In figure 108 similar segments may be seen, one of them showing a longitudinal split. The element x is present, but the nucleolus has disappeared. In many cases the split, if it appears at all, closes quickly and the chromosome bends in U-shape, as in figure 109, plate IV. This figure also shows two centrosomes (c) . In other cases the split persists as in figure 110 and leads to the formation of crosses of a tetrad character (figs. 111, 112, 113) , as in Stenopelmatus and many other insects. Figures 114 to 117 show later stages of the U-shaped chromosomes. Perfect rings are rare. ”

“ Usually the two daughter nuclei are formed in an undivided cell. The resting-stage between the two divisions is only partial. The nucleolus appears and divides into two (figs. 33-36) , and the chromosomes change into the dyad form (fig. 36) , in which they come into the second maturation spindle (figs. 37, 38) . The equatorial plate again shows 26 chromosomes (fig. 39) . The formation of the spermatozoa is peculiar in that the original spermatocyte cell-body, as a rule, does not divide; but the four nuclei resulting from the two maturation divisions develop into sperm-heads in one cell. ”

“ The matter of which an organic cell is composed is found to be not simply a homogeneous, or slightly granular, mass of protoplasm. On the contrary, it appears to be traversed in every direction by delicate fibers, which form an intricate network or reticulum. The interstices of this network are occupied by a fluid or semi-fluid substance of homogeneous appearance, though occasionally containing a few small granules. The reticulum occurs not only in the outer cell, but also within the nucleus, and its fibers extend to and are apparently connected with the nucleolus. ”

“ The nucleus is generally rather more strongly compressed in the direction of the main axis than the capsule itself. The membrane of the vesicular nucleus is thin, but firm, and encloses a clear or finely granular mass of nuclein. The number and size of the contained nucleoli are variable even in one and the same species, and stand in inverse ratio to each other, an obvious result of the gradual process of division. Commonly {xl}from twenty to fifty roundish or spherical, strongly refracting nucleoli, are present; more rarely there are several hundred very small ones. ”

“ When examined under favorable conditions in its passive or resting state, the nucleus is seen to be bounded by a membrane which separates it from the cell plasm and gives it the characteristic sharp contour. It contains an apparently structureless nuclear substance, nucleoplasm or enchylema, in which are embedded one or more extremely minute particles called nucleoli, along with a network of exceedingly fine threads or fibers, which in the active living cell play an essential part in the production of new nuclei within the cell. ”

“ The chromatin-granules are most abundant at two periods in the life of the cell, (1) when it is young and fresh from division, and (2) at the term of its life, when it is itself preparing for division. In the interim they are fewer, smaller, and stain less intensely. In many Protista the whole or greater part of the chromatin is densely aggregated into a central "nuclein-mass" or karyosome {25}suspended in the linin network (long regarded as a mere nucleole) . Such a nucleus is often termed a "vesicular nucleus". [36] ”

“ New nuclei may be formed by their growth and coalescence, the original nucleus sometimes disappearing more or less completely.
In certain cases the division of the nucleus is not followed by that of the cytoplasm, so that a plurinucleate mass of protoplasm results: this is called an "apocyte"; and we find transitional forms between this and the uninucleate or true cell. Thus in one species of Amoeba (A. binucleata) there are always two nuclei, which divide simultaneously to provide for the outfit of the daughter-cells on fission. ”

“ Other as simple but larger organisms may consist of a minute mass of semi-fluid protoplasm, containing granules, as we find to be the case in the plant Vaucheria, [85] and many other Algæ, and in the animal Amœba primitiva. [86]
An organism of this simplest kind or a fragment of a higher organism which presents this simplest condition is called a cell. [87] Very generally [Pg 326] such cell has within it a more or less distinctly marked generally denser and spheroidal body called a nucleus, within which, again, other minute spots may appear called nucleoli. ”