Gilbert Charles Chubb

“ The variation in the form and structure of the cell is an expression of that universal economic law of nature, “division of labour,” with its almost invariable accompanying “morphological differentiation”; the earliest and most fundamental example being in the differentiation of the cell-protoplasm into cytoplasm and nucleus. In multicellular individuals the division of labour to which the structural complexity of the organism is due is between the individual cell-units, some cells developing one aspect, some another, of their vital attributes. ”

“ The influence of the “cells” upon one another, the subordination of the cell’s growth, division and differentiation, to the requirements of the whole organism—seen in normal growth, but nowhere more strikingly than in development and regeneration,—is, however, very difficult of explanation in terms of the cell-theory as this was, until quite recently, generally understood. The very elaborate regional differentiation of the protoplasm often seen in the Protozoa sufficiently indicate that multicellular structure is no essential condition for complex regional differentiation. ”

“ Present Position of the Cell-theory.—Since the time of Schleiden and Schwann a wealth of evidence has accumulated in support of the “cell-theory”—the theory which regards the cell as the unit of organic structure. “The organism consists morphologically, of cells, and subsists, physiologically, by means of the ‘reciprocal action’ of the cells,”—this was the cell standpoint of Schleiden and Schwann, and it is no exaggeration to say that this same conception has dominated the cell-theory almost to the present day. ”

“ Differentiation among the Chromosomes.—If we grant the assumption of a persistent individuality for the chromosomes, then it becomes possible to consider whether in one and the same nucleus these structures may not take varying parts in controlling the cell’s activity in development and in inheritance. Such a differentiation among the chromosomes would be due to independent ancestry rather than to the economy resulting from a division of labour ”

“ It is possible to suggest a mechanico-physical explanation of multicellular structure which will deprive the cell of much of its assumed significance as a unit of organization. The fact that surface area becomes relatively less extensive as bulk increases would alone set a limit to the size of “unicellular” organisms; for not only is there a constant reaction between nucleus and cytoplasm through the nuclear membrane, but the surface of the cell serves both for the intake of food and the elimination of waste material. ”

“ 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. ”

“ Thus one cell specializes in, say, secretion, another in contractility, another in receiving and carrying stimuli, and so forth, so that we have the gland cell, the muscle cell, and the nerve cell, each appropriately grouped with its fellows to constitute the particular tissue or organ—gland, muscle or brain—which has for its function that of its constituent cells. ”

“ 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. ”

“ In unicellular animals we also find division of labour and its accompanying morphological differentiation, but here there is no subdivision of the protoplasm of the organism into the semi-autonomous units which so greatly facilitate division of labour in the Metazoa; instead, division of labour must be between different regions of protoplasm in the single cell. ”

“ The growth of the chromatin is an indispensable prelude to its reproduction (Boveri’s Law of Proportional Growth) . The chromatin is therefore incapable of undergoing reproductive fission in two successive mitotic divisions when these are not separated by a resting (i.e. growth) period. In addition to this, the “bipolar” condition of the adult chromosomes, which determines its mode of attachment to mantle fibres from both poles of the spindle, is not possessed by the unripe chromatin. ”

“ 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”) . ”

“ Since quick movement is usually (but not always) associated with voluntary control, these striated muscle cells are often termed “voluntary” muscle fibres. The great increase in length of these cells is accompanied by the fragmentation of the originally single nucleus.
(b) Cell-modification in Relation to Secretion.—Just as the complex movements considered above were the result of a great development of the power of spontaneous movement possessed by all protoplasm, so cell-secretion is the result of a development of the metabolic processes underlying all vital phenomena. ”

“ The essence of fertilization is, therefore, the union of two germ nuclei only differing from one another in that they are derived from separate individuals. [43] Since the number of chromosomes appearing in mitosis is solely dependent on the number which originally entered into the composition of the nucleus (Boveri’s Law of Chromosome-Constancy) , it follows that, in the mitotic figures of the developing embryo, the chromosomes will be half maternal, half paternal in origin ”

“ This epoch-making work ranks with that of Schleiden in its stimulating influence on biological research, and in spite of the greater technical difficulties in the way, raised animal cytology at one blow to the position already, and so laboriously, acquired by plant cytology. In the animal cell it is the nucleus and not the cell-wall that is most conspicuous, and it is largely to the importance which Schwann, following the example of Schleiden, attached to this structure as a cell constituent, that the success and far-reaching influence of his work is due. ”

“ Centrosome. [25] —This minute body was first detected at the spindle poles by Flemming in 1875, and independently by P. J. van Beneden in 1876. The important part played by the centrosome in fertilization, [26] first described by van Beneden and Theodor Boveri in their papers of 1887–1888, together with the behaviour of this structure in mitosis, led these authors to regard the centrosome not only as the dynamic centre of the cell but as a permanent cell-organ, which, like the nucleus, passed by division from one cell-generation to the next. ”

“ Each cell exhibits in greater or less degree “irritability,” or the power of responding to stimuli; and finally each cell, at some time in its life, is capable of reproduction. It is evident therefore that in the multicellular forms all the complex manifestations of life are but the outcome of the co-ordinated activities of the constituent cells. ”

“ This fibrillation is foreshadowed in the contractile regions of many Protozoa, e.g. in the cirri of hypotrichous Infusoria, the tentacle of Noctiluca, and the myophane layer of Gregarines. In the quickly contracting muscle cell of Vertebrates and insects, further specialization has produced a structure of considerable complexity (fig. 1, b) . Here also the cell is fibrillated, but the fibrillae (sarcostyles) are much more distinct, and are segmented in a manner which gives to the entire cell a “cross striated” appearance. ”

“ Herla and Zoja showed that if the egg of Ascaris megalocephala (var. bivalens) , which possesses two chromosomes, be fertilized with the spermatozoon of var. univalens, in which the germ cell has only one chromosome and that smaller than either of the two in the other variety, three chromosomes reappear, two large and one small, in the cleavage divisions of the resulting hybrid embryo. Zur Strassen’s observations on the giant embryos of Ascaris also support Boveri’s theory. These embryos arise by the fusion of eggs, either before or after fertilization. ”

“ The same economy which, working under the control of natural selection, has produced the complexly differentiated tissues of the higher metazoa, which has led to the sexual differentiation between the conjugating gametes and thus to the sexual differentiation of the parents, has resulted in the production of mitosis. ”

“ So also the problems of development and inheritance have shown themselves to be cell problems, while the study of disease has produced a “cellular pathology.” The most important problems awaiting solution in biology are cell problems. ”

“ In many cases the tetrads (i.e. split chromosomes associated in pairs) have the form of rings, the genesis of which was first clearly determined by vom Rath (1892) in the mole cricket Gryllotalpa (fig. 11) . In this form the sister diads remain united by their ends but widely separate in the middle (fig. 11, b) . As in Cyclops, the belated transverse segmentation appears as the condensation of the chromatin proceeds (fig. 11, d) , but the symmetrical tetrads which this process here produces make it impossible to determine at which of the two divisions reduction is effected. ”

“ Usually indeed the specialization is only rendered evident by the appearance of the formed secretion, e.g. mucus-secreting epithelial cells (fig. 2, b) , the ovarian ovum and the fat cell (fig. 1, a) . In some cases a distinct fibrillation of the cytoplasm accompanies or precedes the appearance of the cell-secretion (Mathews, pancreas cell of Amphibia) . In many cases the internal secretion is no mere accumulation, e.g. ”

“ The manner in which this “reduction” in the number of chromosomes in the germ-cells is brought about, and the significance to be attached to the process, constitute the most hotly debated questions in cytology. In all the metazoa the phenomenon of reduction is associated with the two last and, usually, rapidly succeeding “maturation” divisions by which the definitive germ-cells—ova or spermatozoa—are produced. [46] ”

“ The stimulus conveyed by the nerve fibre to the muscle is out of all proportion to the amount of work it may cause the muscle to do. Although protoplasmic irritability is thus incapable of a simple mechanical explanation, science has rejected the assumption of a special “vital force,” and interprets protoplasmic response as being a long series of chemico-physical changes, [13] initiated, but only initiated, by the original stimulus ”

“ Fig. 8.—Preparation for Mitosis. a, Nucleus of “1/2 blastomere” of Ascaris megalocephala bivalens in resting condition; b and c, nuclei from sister 1/2 blastomeres in preparation for mitosis.
In the same worm (var. univalens) Boveri (1888 and 1899) found that occasional abnormalities in maturation resulted in the suppression of the first polar body and the inclusion of its chromosomes in the second maturation spindle; the egg-nucleus at the time of fertilization thus having two chromosomes instead of one, while the spermatozoon nucleus has only one. ”

“ Entwicklungsgeschichte der wirbellosen Tiere, by permission of Gustav Fischer.
b, Branched nucleus of the “nutritive” cell, from a portion of an ovarial tube of Forficula auricularia.
Striking modifications resulting from specialization in secretion are frequently presented by the nucleus. In many secreting cells this structure is extensively branched, e.g. many gland cells and ovarian nutritive cells of insects (fig. 4, b) . In some cases the nucleus of the gland cell contains a persistent spireme thread (fig. 4, a) ”