Biological membrane

“ The membranogenous substance, the chloride of calcium, diffuses uniformly on all sides from the solid nucleus, and forms an osmotic membrane where it comes into contact with the solution. This spherical membrane is extended by osmotic pressure, and grows gradually larger. Since the area of the surface of a sphere increases as the square of its radius, when the cell has grown to twice its original diameter, each square centimetre of the membrane will receive by diffusion but a quarter as much of the membranogenous substance. ”

“ More convincing than these are membraneous pieces, which may be obtained of various sizes. (See figs. 5, 6.) I have a drawing of such a fragment, which, if enlarged as figs. 1 and 2, would cover over one-quarter of the page, and to which a number of nuclei, more than six, are attached. As a rule, this membrane will, by the manipulations with the cover-glass, not be spread out, but rather folded up, huddled together inclosing some of the gland cells, forming an indescribable nest. ”

“ In many instances this membrane is formed of very thin flattened cells, in other instances it is apparently a homogeneous membrane, and according to some observers is simply a modified part of the basal surface of the cell, while according to others it is a definite structure distinct from the epithelium.
In the secretory portion of the gland and in the smaller ducts the epithelial layer is one cell thick only. In the larger ducts there are two layers of cells, but even here the surface cell usually extends by a thinned-out stalk down to the basement membrane. ”

“ The membrane of cellulose, in which the protoplasm wraps itself up, not only prevents the simplest vegetable organism from moving, but screens it also, in some measure, from those outer stimuli which act on the sensibility of the animal as irritants and prevent it from going to sleep. ”

“ A germ which has undergone no developmental modifications, consists of a spheroidal group of homogeneous cells. Universally, the first step in its evolution is the establishment of a difference between some of the peripheral cells and the cells which form the interior—some of the peripheral cells, after repeated spontaneous fissions, coalesce into a membrane; and by continuance of the process this membrane spreads until it speedily invests the entire mass, as in mammals, or, as in birds, stops short of that for some time. ”

“ When the elaboration of the substance to be secreted is more active, the mechanism of the secretion is modified. The basal membrane of the platform may then be raised at several places at once; instead of a single vesicle projecting into the intestinal cavity, each cell may present a great number more or less voluminous. If all remain small and rapidly detach themselves from the glandular cell, the filaments of the platform are simply separated from each other at different points of the free face, as in Fig. ”

“ All we can say is that certain membranes are more permeable to water than to the substances in solution, and are moreover very unequally permeable to the various substances in solution. As a rule a membrane is much more permeable to a solute whose molecule is of small dimensions. Molecules of salt, for instance, pass through such a membrane much more quickly than do those of sugar. ”

“ There are two germinal membranes, respectively representing the Instrumental and Alimental Systems. Each membrane differentiates, by different appropriations of the yolk substance, into three primary layers, epithelial, neural, and muscular. In the epiblast, or upper membrane, these layers represent: 1°, the future epidermis with its derivatives—hair, feathers, nails, skin glands, and chromatophores; 2°, the future nervous tissue; 3°, the future muscular tissue.130 (Bone, dermis, connective tissue, and blood-corpuscles are subsequent formations.) ”

“ Hence, after a time, the membrane will not be sufficiently nourished by the membranogenous substance, it will break down, and an aperture will occur through which the interior liquid oozes out, forming in its turn a new {125}membranous covering for itself. This is the explanation of the fact that all living organisms are formed by colonies of microscopical elements, although we must not forget that Nature often produces similar results in different ways. ”

“ Those organisms which possess the latter are a little higher in the scale of life than those which remain unclothed by it, but a comparison of the behaviour of the two quickly enables us to say that the membrane is of but secondary importance, and that for those which possess it, it is nothing more than a protective covering for the living substance. ”

“ May it not rather be that our much vaunted knowledge of osmosis is not yet complete? It is quite easy to understand why a dead and a living membrane should behave differently in relation to substances that are passing through them. The molecules of the dead membrane are, comparatively speaking, passive and stable; the molecules in a membrane made of living cells are in a constant state of chemical integration and disintegration; they are the most unstable molecules we know. ”

“ But some cells, especially young ones, are entirely without the enveloping membrane, or stiff wall. Hence we now generally describe the cell as a living, viscous particle of protoplasm, enclosing a firmer nucleus in its albuminoid body. There may be an enclosing membrane, as there actually is in the case of most of the plants; but it may be wholly lacking, as is the case with most of the animals. There is no membrane at all in the first stage. The young cells are usually round, but they vary much in shape later on. ”

“ No doubt they alone do not supply a sufficient basis for any firm conclusions as to the fate of the germinal vesicle. Still, if they cannot sustain, they unquestionably support certain views. The natural interpretation of them is that the membrane of the germinal vesicle is in the act of commencing to atrophy, preparatory to being extruded from the egg, while the contents of the germinal vesicle are about to be absorbed. ”

“ The partial fusion of these membranes makes the Primitive trace, While these three membranes are consolidating into the Primitive trace, the Middle membrane splits into two layers: the Upper terminates in the External blastodermic membrane, the Lower grows gradually around the Internal blastodermic membrane, finally inclosing it. The embryo at this period is a guitar-shaped body (Fig. 177) , consisting simply of three membranes lying over one another, narrowly bound together. ”

“ Response to stimulation thus depends on semipermeability, which implies polarizability of the membranes bounding the irritable cells or elements. This conclusion is one of far-reaching importance, because it localizes the primary change in electrical and hence in other forms of stimulation at the plasma membranes. Some membrane-process forms the first stage of the response to stimulation. The membrane is thus not to be regarded as a mere passive diffusion-preventing barrier between living substance and surroundings, but as the essentially sensitive and controlling portion of the cell. ”

“ The object of this structure is to sweep rapidly, every now and then, over the external surface of the eye, apparently in order to keep the surface clean. But although the membrane occurs in all classes of the sub-kingdom, it is more prevalent in some than in others—e.g. in birds than in mammals. Even, however, where it does not occur of a size and mobility to be of any use, it is usually represented, in animals above fishes, by a functionless rudiment, as here depicted in the case of man. (Fig. 10.)
[75] ”

“ The successive changes in the development were, 1st, a germ membrane visible immediately after the bursting of the vesicle; 2d, at some point an aggregation of granules forming the cumulus of the blastoderma; 3d, the embryo developed lying at this point, as it were upon the membranes, which consist of three superimposed laminæ or layers; 4th, on the serous layer arise the organs of animal life, the brain and spinal marrow, organs of sense, skin, muscles, tendons, ligaments, cartilage, and bone ”

“ Sometimes a number of cylindrical cells are laid end to end, and, by the absorption of the transverse partitions, form a continuous tube, as in the sap-vessels of plants, or in muscular and nervous fibre; and when cells are thus woven together, they are called cellular tissue, which, in the human body, forms a fine net-like membrane, enveloping or connecting most of its structures. In pulpy fruits, the cells may be easily separated one from the other; and within the cells are smaller cells, commonly known as pulp. ”

“ Here the protoplasm of the cells is endowed with the faculty of secreting over its surface a firm, resisting membrane, composed of cellulose, a substance destitute of nitrogen, thus totally different from the contained protoplasm, and incapable of manifesting any of the phenomena of life. ”

“ The animal cell, in its most complete form, is comparable in most parts of its structure to that of the plant, but differs from it in the entire absence of the 'cellulose wall' or of anything that represents it, the cell-contents being enclosed in only a single limitary membrane, the chemical composition of which, being albuminous, indicates its correspondence with the primordial utricle. ”

“ The naked cells which have been alluded to live in water, and call therefore for no differentiation in connexion with this necessity; but those which are surrounded by a cell-wall always develop within themselves a vacuole or cavity which occupies the greater part of their interior, and the hydrostatic pressure of whose contents keeps the protoplasm in contact with the membrane, setting up a condition of turgidity. ”

“ This is done by pulsation of the vacuole, which ultimately bursts, passing fluid waste to the outside. Solid wastes are passed out of the cell in somewhat the same manner. No breathing organs are seen, because osmosis of oxygen and carbon dioxide may take place anywhere through the cell membrane. The nucleus of the cell is not easily visible in living specimens. ”

“ Another method of increasing absorption from the cellular membrane is by warm air, or by warm steam. If the swelled legs of a dropsical patient are inclosed in a box, the air of which is made warm by a lamp or two, copious sweats are soon produced by the increased action of the capillary glands, which are seen to stand on the skin, as it cannot readily exhale in so small a quantity of air, which is only changed so fast as may be necessary to permit the lamps to burn. ”

“ There are membranes in all sanguineous animals. Membrane is like a dense thin skin, but it differs in kind, for it is neither divisible nor extensible. There is a membrane round every bone and every intestine, both in the greater and smaller animals; they are inconspicuous in small animals, owing to their thinness and small size. The principal membranes are two, which surround the brain, one round the bones of the head, and this is stronger and thicker than that round the brain itself; and after these, the membrane which surrounds the heart. ”

“ Those who enumerate three tunics are, I believe, in error, owing to the ancients having described the same membrane at one time under the title of “chorion,” from the concourse of veins, at another under the name of “allantois” from its form.
Unquestionably, every “conception” is inclosed in two envelopes, just as the brain is surrounded by its two membranes; every tree and fruit, moreover, has its double bark; and lastly, seeds and fruits are protected by a double covering, the outermost of which is harder and stronger than the inner one. ”

“ With regard to the former, we have seen that in the single cells, constituting unicellular organisms, there is always a bounding membrane of denser texture than the rest of the protoplasm. As the cell develops its capabilities, we have a shell or case of non-living matter secreted around it, with apertures for communication with the outside world, and increasingly effective protection is provided as protoplasm, whether in the single cell or the body, leaves the water, and has to face the inclemencies of terrestrial life. ”

“ Osmotic membranes behave exactly like colloids. The resistance which they oppose to the passage of different substances varies with the nature of the liquid or solute concerned. There is no real difference between the passage of a solution through an osmotic membrane and its diffusion through a colloid. The protoplasm of a living organism, being a colloid, acts exactly like an osmotic membrane so far as regards the distribution of solutions and substances in solution. ”

“ You know that if substances of a certain kind are placed in solution next to each other and separated only by a thin porous membrane, there will take place and interchange of the molecules or ions of the one substance with those of the other until the two liquids are completely mixed. ”

“ In order to understand the beginnings of life it is important to have a clear conception of [Pg 235] these different sacs, for these two membranes lying within the yolk sac, and together known as the blastodermic membrane, and separately as the outer and inner layers of the blastodermic membrane, contain the anatomical elements from which the organized being with its fully-formed body is to grow. Indeed, it may be said that these two blastodermic membranes are the body of the embryo which is to develop into the fully-formed physical man. ”