Anatomy General Considerations On Cells
cell protoplasm nucleus tissues substance contents wall differentiation minute ovum
ANATOMY GENERAL CONSIDERATIONS ON CELLS The simplest form of organic matter capable of exhibiting the phenomena of life is called Cyto-blastema or Protoplasm. It possesses a viscous or jelly-like consistency. Under the highest powers of the microscope it seems to be homogeneous, or dimly granulated, like a sheet of ground glass. Not only can it assimilate nutriment and increase in size, but it possesses the power of spontaneous movement and contractility. It enters in a very important manner into the structure of the bodies of the lower animals. The elongated processes, or pseudo podia, to which Dujardin applied the name of sarcode, which the Rhizopoda can project from their surface into the surrounding medium, and again withdraw into their substance, consist of protoplasm, and may be cited as furnishing excellent examples of its motive and contractile power. From the recent researches of Haeckel it would appear that protoplasm is capable of forming, without the super-addition of any other structure, independent organisms, which stand at the lowest grade of organisation, and from their extreme simplicity are named by him Monera. To the group Monera belong the genera Protammba, Protogcues, and Bathybius. Of these, Bathybius is that which has attracted most attention. It has been regarded as a layer of soft slimy undifferentiated protoplasm covering the bottom of the deep sea, and capable of exhibiting the phenomena of contractility, growth, assimilation of food, and reproduction. Doubts, however, have been expressed regarding the nature of this Bathybius, so that it cannot now be cited as so definite an organism as the freely-swimming Protamceba and Protogenes. Haeckel has referred these simple organisms to a subkingdom of PROTIST2E, which he considers to he on the confines of both the animal and vegetable kingdoms. To a mass of protoplasm, whether it forms, as in one of these PROTIST4E, an independent organism, or is merely a portion of the substance of the body of a higher organism, he has given the general name of a Cytode. Sometimes a cytode is a naked clump of C soft protoplasm, without a trace of differentiation either on its surface or in its substance, as in the freely-moving Moncra ; at others the peripheral part of the cytode . hardens, and differentiates into a more or less perfect envelope, as in the genera Protomonas and Protomyxa. So far back as 1861, Lionel Beale had described, under the name of germinal matter (Bioplasm), minute living particles of vegetable protoplasm, and in 1863 he demonstrated the presence of extremely minute particles of living matter in the blood. More recently Stricker has also called attention, in the bodies of the higher animals, to minute detached clumps of protoplasm which exhibited the phenomena of life.
As a rule, however, in both vegetable and animal ( organisms the specks or clumps of protoplasm assume definite shapes, and show evidence of an internal differentiation. In the midst of a minute clump of this substance a sharply-defined. body called a nucleus is found, which differs from the surrounding protoplasm in not being contractile; and sometimes a minute speck, or nucleolus, exists within the nucleus. When a definite clump of protoplasm contains a nucleus in its interior, whether a nucleolus be present or not, it is called a Nucleated Cell. Cells are definite anatomical and physiological units, and exhibit all the phenomena of life. Some of the lowest organisms consist merely of a single cell, others of two or more cells united together, and these are called uni- or multi-cellular organisms. Cells also enter in the most material manner into the constitution of the textures of all the higher forms of plants and animals. Not unfrequently the peripheral part of the protoplasm of the cell differentiates into a distinct investing envelope, technically named a cell wall or cell membrane.
In the earlier periods of investigation into the minute structure of cells it was believed that a cell wall was constantly present, and that each cell was a minute microscopic vesicle or bladder, which in its typical shape was globular or ovoid, but capable of undergoing various modifications both in form and chemical composition. The material enclosed by the cell wall was termed the cell contents, and either in the midst of these contents or in contact with the cell wall was the nucleus, which might or might not contain a nucleolus. Schwann believed that the cell wall was the most active constituent of the cell, i.e., possessed the power not only of producing chemical and physical changes in its own substance and in the cell contents, but of separating materials from the surrounding media, - of secreting them, as it were, into the interior of the cell. In this manner he accounted for the formation in seine cells of fat, in others of pigment, in others of the characteristic secretion of glands, and so on.
It was then maintained by John Goodsir that the nucleus was the part of a cell which in all probability was concerned in separating and preparing its characteristic cell contents, and in its nutrition. Martin Barry and Goodsir also contended that the reproduction and multiplication of cells were due to self-division of the nucleus, which was thus the source of successive 'broods of young cells. They gave to the nucleus, therefore, an importance in the economy of the cell greater than had previously been assigned to it.
As the investigations into cell structure became more extended, it was ascertained that a cell wall was by no means always present; that in many of the cells in which it had been supposed to exist it could not satisfactorily be demonstrated, and that in others, more especially in young actively-growing cells, no trace of an investing envelope could be observed. Hence the importance of the cell wall as an essential comp( nent of a cell was still further diminished; and Leydig then defined a cell to be a little mass composed of a soft substance enclosing a central nucleus.
But a most important advance in our conceptions of the essential structure of a cell was made when Briicke pointed out that the contents of cells not unfrequently possessed the property of spontaneous movement and contractility, and when Max Schultze determined that the contractile substance termed sarcode, which forms so large a part of the bodies of the lower animals, was analogous and apparently homologous with the contents of young actively-growing animal and vegetable cells, before a differentiation of these contents into special secretions or other materials had taken place. As the term " protoplasm" had been introduced by Von Mohi to express the contents of the vegetable cell, which undergoes changes in the process of growth, it was adopted by the animal histologist; and Max Schultze suggested that a cell should be defined to be a nucleated mass of protoplasm, - a definition which is adopted in this article. Now, as protoplasm, whether it occurs along with a nucleus in the form of a cell, or in independent clumps or cytodes, exhibits not merely the property of contractility, but the power of growing and maintaining itself, it is regarded as the functionally active constituent of the cell. And thus our conceptions as to the part of the cell in which its functional activity resides have passed through three phases. In the first, the cell wall; in the second, the nucleus; in the third, the protoplasm cell contents, or cell substance, has been regarded as the active constituent, not only as regards its nutrition, but the reproduction of young cells. But though the protoplasm can of itself perform these offices, yet there can be no doubt, as Barry and Goodsir were the first to show, that the nucleus of the cell plays a part not unfrequently in the multiplication of cells by self-division.
One of the most characteristic cells is the mammalian ovum. In it a cell wall exists, known as the zona pellucida or vitelline membrane ; within this envelope is the granular yelk or cell contents, in the midst of which is imbedded the nucleus or germinal vesicle, which in its turn contains the nucleolus or germinal spot. The granules of the yelk are a special metamorphosis of the protoplasm cell substance.
Schwalm made the important generalisation that the tissues of the animal body are composed of cells, or of materials derived from cells, "that there is one universal principle of development for the elementary part of organisms, however different, and that this principle is the formation of cells." The ovum is the primordial or fundamental cell, or germ-cell, from which, after being fertilised by the male sperm, the tissues and organs of the animal body are derived. Within the fertilised ovum multiplication of cells takes place with great rapidity. It is as yet an unsettled question how far the original nucleus of the ovum participates in this process of multiplication ; but there can be no doubt that the protoplasm cell contents divide, first into two, then four, then eight, then sixteen segments, and so on. Each of these segments of protoplasm contains a nucleus - is, in short, a nucleated cell, and the protoplasm of these cells exhibits the property of contractility. The ovum or germ-cell is therefore the immediate parent of all the new cells which are formed within it, and mediately it is the parent of all the cells which, in the subsequent processes of development and growth, are descended from those produced by tne segmentation of the .yelk. The process of development of young cells within a parent cell, whether it occurs in the ovum or in a cell derived by descent from the ovum, is called the endogenous reproduction of cells. But cells may multiply by a process of fission - i.e., a constriction, gradually deepening, may take place in a cell instil it is subdivided into two ; the -nucleus at the same time participating in the constriction and subdivision. A third mode of multiplication of cells is by buckling: little clumps of protoplasm bud out from the protoplasm of the parent cell, become detached, and assume an independent vitality. If a nucleus differentiates in the interior of such a clump, it becomes a cell; if it remains as a mere clump of protoplasm, it is a cytode.
These various methods of multiplication are all confirmatory of Schwann's generalisation of the descent or derivation of cells from pre-existing cells. But as the nucleated cell, either with or without a cell wall, is not, in the present state of science, regarded as the simplest and most elementary unit capable of exhibiting vital phenomena, and as these phenomena can be displayed by individual clumps of protoplasm, without the presence of a nucleus, some modification of the doctrine, as regards the formation of the tissues from nucleated cells, seems to be necessary. For, although there can be no doubt that all the tissues are mediately derived from the ovum or fundamental cell, and that most of the tissues are derived directly from nucleated cells, yet there is reason to think that a differentiation of a cytode clump of protoplasm into tissue may take place, so that the direct formation of such a tissue would be, not from a nucleated cell, but from the more simple cytode. Hence a more comprehensive generalisation, to which observers have gradually been led from the consideration of numerous facts, has now been arrived at, - that the tissues and organs of the body, whatever may be their form and composition, are formed of protoplasm, or produced by its differentiation; and that the protoplasm itself is derived by descent from the protoplasm substance of the primordial germ-cell. Some, indeed, have contended that protoplasm, cells, and their derivatives can arise by a process of precipitation or aggregation of minute particles or molecules in an organic infusion, and that living matter may be thus spontaneously generated. But the evidence which has been advanced in support of this hypothesis is by no means satisfactory or conclusive, whilst the correctness of the theory of the direct descent of protoplasm from pre-existing living protoplasm is supported by thousands of observations made by the most competent inquirers.
In the process of conversion of protoplasm into the several tissues, there takes place a differentiation of form and structure (i.e., a morphological differentiation), and of composition (i.e., a chemical differentiation), as the result of which a physiological differentiation is occasioned, whereby tissues and organs are adapted to the performance of special functions. Hence arise the several forms of tissue which occur in the human body and in the higher animals. Many of the tissues consist exclusively of cells which present in different parts of the body characteristic modifications in external configuration, in composition, and in properties, as may be seen in the fatty tissue, pigmentary tissue, and epithelium. Other tissues, again, consist partly of cells, and partly of an intermediate material which separates the constituent cells from each other. here also the cells present various modifications; and the intermediate material, termed the matrix or intercellular substance, varies in structure, in composition, and in properties in the different textures, as is seen in the connective, cartilaginous, osseous and muscular tissues It is not an easy matter to devise a classification of the tissues, based on their structural characters, which shall be in all respects logically perfect ; but a convenient basis of arrangement for descriptive purposes may be found by dividing them into those which consist - lst, of cells suspended in fluids ; 2d, of cells placed cn free surfaces ; 3d, of cells imbedded in solid tissues.