Cytology

“ The Germplasm Cycle.—Since Weismann’s postulation of the independence of soma and germplasm in 1885 many attempts have been made to trace the path of the hereditary substance from one generation to the next. A recent book by Hegner [181] summarizes the success attained in various groups of animals.
429 Cytology. Another important field of investigation which has attracted many workers is that which pertains to the life of the cell—the science of cytology. Although the celltheory was established as early as 1839, little advance was made in this subject in America before 1880. ”

“ The new science of cytology has made us aware that the division of cells is not a passive or a simple process, but is extremely active, complex and varied. Living protoplasm is in motion, and the discovery that cell walls are not hermetically closed, but are perforated by delicate protoplasmic strands, lends strength to the belief of some biologists that protoplasm circulates, not only inside the individual cells, but through the entire organism. Conjugation may signify that such a circulation extends also throughout the species. ”

“ And yet, in spite of this fact, there are few fields of scientific work possessing more general interest than that of cytology. At the present day the greatest problems of biology are centered in the cell. Assimilation, growth, metabolism, reproduction, differentiation, inheritance and variation—these are at bottom cellular phenomena, the result of the structure and functions of cells. ”

“ In short, the field was open, the race was on, and it was a matter of self-restraint that a man did not enter and strive long and well for some anatomical prize. This branch of botany is still alive, and never more so than to-day, when cytology offers many attractive problems for the cytologist. ”

“ It is (vide fig. 35A, g.e.) composed of two or three layers of cells, with granular nuclei, of which the outermost layer is more columnar than the remainder, and has elongated rather than rounded nuclei. Its cells, though they vary slightly in size, are all provided with a fair amount of protoplasm, and cannot be divided (as in the case of the germinal epithelium of Birds, Elasmobranchii, &c.) , into primitive ova, and normal epithelial cells. Very occasionally, however, a specially large cell, which, perhaps, deserves the appellation primitive ovum, may be seen. ”

“ The whole of the interior of each of the formative cells, and of the other cells which are enclosed between the epiblast and the hypoblast, become converted into new cells. These are the cells of the mesoblast. I have not been able perfectly to satisfy myself as to the exact manner in which this takes place, but I am inclined to think that some or all of the spherules which are contained in the original cells develop into nuclei for the new cells, the protoplasm of the new cells being formed from that of the original cells. ”

“ The granular deeply staining cytoplasm, the clear non-staining nucleus with its bright nucleolus and the nucleolus-like yolk nucleus, all show beyond doubt that these freely floating cells originate in the ovary.
In some of my preparations these cells are found not only floating free, but wandering through the tissues. Fig. 70 shows two such wandering cells fixed just as they were making their way either through the digestive epithelium into the gelatine of the floor of the stomach, or from the gelatine into the epithelium. ”

“ 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 one outstanding discovery which has placed the science of Embryology on an absolutely firm basis, and which has made clear so many of the facts, which were previously puzzling, is this: that the germ-cells which give rise to new individuals are themselves produced from pre-existing germ-cells. The entire embryo, or young infant, is derived from one single cell which we have called the fertilised ovum, and that in its turn was derived from the union of two germ-cells, one from the male parent, and one from the female. ”

“ As soon as a spermatozoon enters an ovum, it disappears and is completely absorbed, and, as the ovum in turn is instantly possessed of new powers, the result of this union is a cell which was previously non-existent.
This new cell is not only capable of reproduction by means of segmentation or cell division, but in the course of its sub-division and proliferation, it forms groups of cells which develop into tissues and structures widely different from each other. The entire complex human body, in addition to the placenta, cord, and membranes, arises from the single, extraordinary cell. ”

“ I. Each human individual, like every other higher animal, is a single simple cell at the commencement of his existence.
II. This “stem-cell” (cytula) is formed in the same manner in all cases—that is, by the blending or copulation of two separate cells of diverse origin, the female ovum and the male spermatozoon.
III. Each of these sexual cells has its own “cell-soul”—that is, each is distinguished by a peculiar form of sensation and movement.
IV. At the moment of conception or impregnation, not only the protoplasm and the nuclei of the two sexual cells coalesce, but also their “cell-souls” ”

“ The number of living investigators in histology is enormous; and their work in the subject of cell-structure and in the department of embryology now overlaps.
In pathological histology may be observed an illustration of the application of biological studies to medicine. While no attempt is made to give an account of these practical applications, they are of too great importance to go unmentioned. Histological methods are in constant use in clinical diagnosis, as in blood counts, the study of inflammations, of the action of phagocytes, and of all manner of abnormal growths. ”

“ Inasmuch as the idea of an organic cell, as now adopted by histologists, rests on the presence of two different essential parts in this elementary organism, the internal cell and the external protoplasm, we must distinguish also two different kinds of elementary organisms: germless cytods, as moners for example, and the real germ-inclosing cells, which originate from the former by secreting in the interior of the small mass of protoplasm a true germ or nucleus. ”

“ 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. ”

“ Most cells can do no more than produce units like themselves. The bodies of women contain, however, a type of cell which possesses a far more wonderful power. Provided the requisite conditions for such development are met, these cells are capable of developing into human beings. Each of these remarkable units is called an Ovum, or egg-cell, and represents one variety of the germinal cells. But the other variety, represented by the Spermatozoon and developed only in the male sex, is also required for the production of a human being.
Every ovum originates in the ovaries. ”

“ The cellular pathology is the pathology of to-day; indeed, protoplasm—its vital characteristics under abnormal influences and its decay—will be regarded most likely as the basis of pathology in all time. According to our present knowledge of physiological and pathological processes, we must regard the cell as the ultimate biological unit—a unit of structure and a unit of function ”

“ Another theory is that the tumor is the result of imperfect embryonic development. The development of the child from the ovum is the result of a continued formation and differentiation of cells. A cell mass is first produced, and the cells in this differentiate into three layers called ectoderm, entoderm and mesoderm, from which the external and internal surfaces and the enclosed tissues respectively develop, and the different organs are produced by growth of the cells of certain areas of these layers. ”

“ The seminal elements of the male are also only simple cells, and the entire mysterious process of fructification is after all nothing but the fusion or concrescence of two different cells, the one a female egg-cell, and the other a male semen-cell. In consequence of this fusion the germs of the two combined cells dissolve, and therewith the young, newly-generated individual begins his existence as a simple cytod, or a small germless ball of protoplasm. ”

“ Equal conjugation was demonstrated fully by F. Schaudinn in Actinophrys; two individuals approach and enter into close contact, and are surrounded by a common cyst wall. The nucleus of either male divides; and one nucleus passes to the surface at either side, and is budded off with a small portion of the cytoplasm as an abortive cell ”

“ The siliceous cell-wall is covered on the outside by a layer of protoplasm called the coleoderm. This layer may be quite thin and evident only when treated with fuchsin or Bismarck brown, or it may be of considerable thickness. The cell contains the cytoplasma, protoplasm, cell-sap, endochrome, pyrenoids, oil globules and nucleus, together with certain other less understood bodies. ”

“ Each fresh cell-division sorts out tendencies which were mixed in the nucleus of the mother-cell, until the complete mass of embryonic cells is formed, each with a nuclear idio-plasm which stamps its specific histological character on the cell. ”

“ 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 the free state the cytoblasts increase rapidly until they attain a certain size, when they are surmounted by a fine diaphanous bubble; this is the young cell, at first a segment of a sphere, its plane side formed by cytoblasts, and its convex side by young cells (the cell-epidermis) similar to a watch-crystal on a watch. Gradually the bubble expands, becomes more consistent, and the wall is composed of cytoblasts and of a gelatinous substance. ”

“ Sometimes the cells loose their hold and are swept on again, but in other cases a minute bud of protoplasm will appear on the other side of the wall of the vessel, opposite to the spot where the leucocyte is clinging, and as this grows larger, a narrow neck of protoplasm can be traced through the wall directly to the leucocyte, and presently the mass of the leucocyte becomes proportionately smaller as the external bud of protoplasm grows larger. ”

“ Questions of heredity and variation are cytological ones—that is, questions of the anatomy, physiology, physiological chemistry, and pathology of cells. The most important part of a cell, as far as these questions are concerned, is the nucleus. ”

“ Until, however, further evidence is forthcoming in support of this syncytial theory of structure, it would be unwise to regard it as established sufficiently to constitute a serviceable working hypothesis; hence, for the time being, we must accept the assertion that the cell represents the ultimate tissue-unit. Our present day definition of a cell is a minute portion of living organized substance or protoplasm. ”

“ The germinal wall certainly gives rise to the hypoblast cells, which mainly grow at its expense. They arise at the edge of the area pellucida, and when first formed are markedly columnar, and enclose in their protoplasm one of the smaller vesicles of the germinal wall. ”

“ The new cell is not formed by a destruction of the old one, but directly from it by a process of metamorphosis. These hypoblast cells are formed first at the centre and later at the circumference, so that from the first the cells at the circumference are less flattened and more granular than the cells at the centre. A number of cells of the original lower layer are enclosed between this layer and the epiblast ”

“ On the contrary, it is supposed that both formative and formed material are one in kind—that [183] all the cellular tissues of a multicellular organism, like the single cell of a unicellular organism, are per se endowed with the vital property of self-multiplication; and that whether this property finds its expression in normal growth, in abnormal increments of growth (such as tumours) , in processes of repair, in the various forms of a-sexual reproduction, or in the more specialized form of sexual fertilization, there is everywhere an exhibition of one and the same capacity. ”

“ I. fig. 45 a) ; and it will be noticed that the new cells are formed at the end of the embryonal vesicle, opposite to that situated at the apex of the embryo-sac. As the cell-division and formation proceed further, a mass of new cells is produced (Pl. I. figs. 46 c{47} & 47) , forming the rudimentary embryo; and from this, by further growth, the perfected embryo (fig. 55) results; or, to use a fashionable technical term, the simply cellular embryonic mass becomes differentiated into the radicle, cotyledons, and plumule, forming the embryo. ”