Chromatin

“ By double or partial fertilization, different regions in the same embryo may show nuclei of different sizes (Boveri) . We must therefore distinguish in the cell between “young” and “adult” chromatin. In other words the chromatin must be regarded as being composed of individual units, each with a definite constant structure and maximum growth (Boveri, 1904) . This conclusion is strongly suggested, not only by the evidence in favour of the individuality of the chromosomes considered above, but also by the independent reproductive activity of the chromatin granules in the prophase of mitosis. ”

“ It is indeed usually assumed that the mitotic mechanism is not merely for the distribution, but for the equal distribution, of the sister granules to the two daughter nuclei. The conspicuous part the chromatin is seen to play in the whole mechanism of heredity—in maturation, fertilization and development—indicating as it does that the chromatin is the chief, if not the only, bearer of the specific qualities of the organism, sufficiently clearly emphasizes the importance of the equal distribution of this substance between the daughter cells at successive cell-divisions. ”

“ While the unstable units of chromatin, ever undergoing changes, diffuse energy around, they may also be units which, under the conditions furnished by fertilization, gravitate towards the organization of the species. Possibly it may be that the complex combination of proteids, common to chromatin and cytoplasm, is that part in which the constitutional characters inhere; while the phosphorized component, falling from its unstable union and decomposing, evolves the energy which, ordinarily the cause of changes, now excites the more active changes following fertilization. ”

“ The parts on which we wish particularly to rivet our attention are the densely stained substances scattered along or embedded in the strands of this network in irregular granules and patches. This substance is called chromatin. It takes its name from the fact that it shows great affinity for certain stains and becomes intensely colored by them. This deeply colored portion of the cell, the chromatin, is by most biologists regarded as of great importance from the standpoint of heredity. ”

“ The chromatin of the fibrils has aggregated at the poles and the equator of the nucleus, and left apparent intermediate lines of achromatin.
In vegetable-cell divisions Strasburger finds none of this regular process, but only a vague approach to it in the movements and aggregations of masses of chromatin. But the achromatin-striæ of the nuclear spindle, the equatorial plate, and the sun-like polar rays, are well declared. In some cases of abnormally rapid nutrition a threefold division takes place, and possibly a still greater number of new cells may be formed. ”

“ When the time for fission is approaching this dispersed chromatin begins to gather itself together: reaching its eventual concentration through several stages. By its concentration are produced the chromosomes, constant in number in each species of plant or animal. It is alleged that the substance of the chromosomes is not continuous, but consists of separate elements or granules, which have been named chromomeres ”

“ Observation of the process of division of the nucleus (karyokinesis) in plants generally has shown (for details see Cytology) that the linin-reticulum of the resting nucleus breaks up into a definite number of segments, the chromosomes, each of which bears a series of minute bodies, the chromatin-disks or chromomeres, consisting largely of a substance termed chromatin. In the ordinary homotype divisions of the nuclei the characteristic number of chromosomes is always observable; but when the spore-mother-cells are being formed the number of chromosomes is reduced to one-half. ”

“ At the close of the spermatogonial divisions, when the disruptive processes reduce the other chromosomes to masses of chromomeres in which chromosome identities are not apparent, the accessory chromosome, with apparently more cohesive vigor than the others, retains its general form and is at all times distinguishable. It is marked off from the others, not only by persistence of form, but also by the difference in staining reaction, this being such as is usually exhibited by chromatin when concentrated into homogeneous masses. ”

“ The conception of chromatin is one based upon its relations to the vital activities and life cycle, as a whole, of the organism or cell, and not upon any definable material, that is chemical and physical, properties.
The importance of protoplasm, as the physical and material basis of life, has caused it to be the subject in recent years of much minute and laborious research. It seems obvious that matter so peculiarly endowed must possess a complexity of structure and organization far exceeding that which at first sight meets the eye. ”

“ In the different propositions I have quoted, we [Pg 36] have to deal with what is merely a fallacy in rhetorical disguise. For, from the premiss that the chromatin has the power of impressing specific character upon the protoplasm of the cell, it by no means follows that two cells, distinguishable by the nature of their plasma-products, must therefore contain different kinds of protoplasm. There are other possibilities to be reckoned with. ”

“ When a cell divides, the readily stainable bodies or chromosomes, present in definite number within the nucleus, are divided, usually by a most intricate process, in such a manner that equal amounts are bequeathed by the mother-cell to each of the two daughter-cells. Spencer favoured the view that the chromatin, which "consists of an organic acid (nucleic acid) rich in phosphorus, combined with an albuminous substance, probably a combination of various proteids" may be peculiarly unstable and active. ”

“ 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. ”

“ In the yeast cell the nucleus is represented by a homogeneous granule, probably of a nucleolar nature, surrounded and perhaps to some extent impregnated by chromatin and closely connected with a vacuole which often has chromatin at its periphery, and contains one or more volutin granules which appear to consist of nucleic acid in combination with an unknown base. Some observers consider that the yeast nucleus possesses a typical nuclear structure, and exhibits division by mitosis, but the evidence for this is not very satisfactory. ”

“ 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. ”

“ An animal or plant in which the chromatin is unequally divided among the cells, must have tissues of uncertain formation. Assume that an organ has, by survival of the {264}fittest, been adjusted in the proportions and qualities of its parts to a given function. If the multiplying protoplasts, instead of taking equal portions of chromatin, have some of them smaller portions, the parts of the organ formed of these, developing less rapidly and having inferior energies, will throw the organ out of adjustment, and the individual will suffer in the struggle for life. ”

“ In the cell division the chromosomes no longer split into equivalent halves, but some characters are portioned off to some cells and others to other cells. Those cells which are to carry on digestive functions when they are formed receive chromatin material which especially controls them in the performance of this digestive function, while those which are to produce sensory organs receive a different portion of the chromatin material. Thus the adult individual is built up as the cells receive different portions of this hereditary substance contained in the original chromosomes. ”

“ The granules have been breaking up into finer and finer pieces, until, a little after the cell has reached its maximum, the chromatin has largely disappeared, or, to be more [50] exact, has lost its affinity for stains. (Fig. 16.) As the cell begins to elongate, the nucleus goes to one end, namely, that toward the periphery of the cyst. (Figs. 14 and 15.) The chromatin again frequently appears in patches as the cell decreases in size. (Figs. 33 and 36.) In many cells the patches never disappear. ”

“ The chromosomes of the second spermatocytes break up and the chromatin becomes diffused all through the nucleus. Later the chromatin collects in granules again and finally forms the walls of the tube-like spermatozoon head.
2. The spindle fibers break loose as the clear space is formed around the chromatin mass. They unite and contract, becoming fewer, thicker, and shorter. These bend and form the “striated nebenkern.” The fibers break up and sometimes show a blackberry appearance. ”

“ Whatever this difference may be due to,—whether, for instance, to actual differences of permeability, or possibly to differences in “surface-charge,”—the fact is that the chromatin substance now behaves after the fashion of a “diamagnetic” body, and is repelled from the stronger to the weaker parts of the field. ”

“ The chromatin stands towards the other contents of the cell in the same relation that a nerve-element stands to any element of an organism which it excites." "We may infer that cell-evolution was, under one of its aspects, a change from a stage in which the exciting substance and the substance excited were mingled with approximate uniformity, to a stage in which the exciting substance was gathered together into the nucleus and finally into the chromosomes, leaving behind the substance excited, now distinguished as cytoplasm." ”

“ Every nucleus consists of a membrane by which it is separated from the protoplasm, and three internal constituents: First, a network of living material, more or less intermingled with which is a second special substance, chromatin, which owes its name to the very marked affinity which it displays for the various artificial coloring matters which are employed, in microscopical research. ”

“ Weismann decides boldly—but again without bringing forward proof—in favour of the former interpretation. 'For the chromatin,' he remarks, [10] 'cannot become different in the cells of the fully formed organism; the differences in the chromatin controlling the cells must begin with the development [Pg 38] of the egg-cell and must increase as development proceeds; for otherwise the different products of the division of the egg-cell could not give rise to entirely different hereditary tendencies. ”

“ Recent observations on the reproduction of some Sarcodina have shown that the chromidia may possess great importance in the life-cycle as representing generative chromatin which, like the micronucleus of the Infusoria mentioned above, remains in reserve until, by the process of syngamy, the nuclear apparatus is renewed; while the principal nuclei represent, like the macronuclei, somatic or vegetative chromatin which becomes effete and is cast off or absorbed when syngamy takes place. ”

“ Nägeli has clearly shown that we must assume a stable, that is, a solid molecular architecture. There thus remains only the nuclear reticulum with its chromatin granules, and when we remember what we have learnt of the behaviour of this chromatin substance during division and amphimixis we can entertain no doubt that the sought-for bearer of the inheritance is contained in the substance of the chromosomes. ”

“ Assuming it to be true that the chromatin is the important part of the cell in the matter of heredity, then it follows that we have two kinds of spermatozoa that differ from each other in a vital matter. We expect, therefore, to find in the offspring two sorts of individuals in approximately equal numbers, under normal condi­tions, that exhibit marked differences in structure. ”

“ In films which are properly stained with Wright's fluid the young organisms are small, round, ring-like or irregular, sky-blue bodies, each with a very small, sharply defined, reddish-purple chromatin mass. Many structures—deposits of stain, dirt, blood-plaques lying upon red cells, etc.—may simulate them, but should not deceive one who looks carefully for both the blue cytoplasm and the reddish-purple chromatin. ”

“ In addition to the chromatin there may be present in the nucleus one or more, usually spherical, and as yet somewhat enigmatical bodies, the “nucleoli.” In addition to the nucleus and cytoplasm, a third body, the “centrosome,” has often been considered as a constant cell-structure. It is a minute granule, usually lying in the cytoplasm not far from the nucleus, and plays an important part in cell-division and fertilization (see below) .
Cell-differentiation.—Both among unicellular and multicellular individuals the cell assumes the most varied forms and performs the most diverse functions. ”