THE CELL-MEMBRANE From a general point of view the cell-membrane or intercellular substance is of relatively minor importance, since it is not of constant occurrence, belongs to the lifeless products of the cell, and hence plays no direct part in the active cell-life. In plant-tissues the membrane is almost invariably present and of firm consistency. Animal tissues are in general characterized by the slight development or absence of cell-walls. Many forms of cells, both among unicellular and multicellular forms, are quite naked, for example Amcrba and the leucocytes ; but in most, if not in all, such cases, the outer limit of the cell-body is formed by a more resistant layer of protoplasm —the " pellicle " of Butschli — that may be so marked as to simulate a true membrane, for example, in the red blood-corpuscles (Ranvier, Waldeyer) and in various naked animal eggs. Such a " pellicle " differs from a true cell-membrane only in degree ; and it is now generally agreed that the membranes of plant-cells, and of many animal-cells, arise by a direct physical and chemical transformation of the peripheral layer of protoplasm. On the other hand, according to Leydig, Waldeyer, and some others, the membrane of certain animal-cells may be formed not by a direct transformation of the protoplasmic substance, but as a secretion poured out by the protoplasm at its surface. Such membranes, characterized as " cuticular," occur mainly or exclusively on the free surfaces of cells (Waldeyer). It remains to be seen, however, how far this distinction can be maintained, and the greatest diversity of opinion still exists regarding the origin of the different forms of cell-membranes in animal-cells.
The chemical composition of the membrane or intercellular substance varies extremely. In plants membrane consists of a basis of cellulose, a carbohydrate having the formula ; but this substance is very frequently impregnated with other substances, such as silica, lignin, and a great variety of others. In animals the intercellular substances show a still greater diversity. Many of them are nitrogenous bodies, such as keratin, chitin, elastin, gelatin, and the like ; but inorganic deposits, such as silica and carbonate of lime, are common.