38 GENERAL SKETCH OF THE CELL 



G. 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 mem- 

 brane 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 Aviceba 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 

 " pelhcle " of Blitschli — that may be so marked as to simulate a true 

 membrane, for example, in the red blood-corpuscles (Ranvier, Wal- 

 deyer) 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 periph- 

 eral 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, how- 

 ever, 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 sub- 

 stance varies extremely. In plants membrane consists of a basis of 

 cellulose, a carbohydrate having the formula Q^Yi^^Or^ ; but this sub- 

 stance is very frequently impregnated with other substances, such 

 as silica, lignin, and a great variety of others. In animals the inter- 

 cellular 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. 



H. Polarity of the Cell 



In a large number of cases the cell exhibits a definite polarity, its 

 parts being symmetrically grouped with reference to an ideal orga/ac 

 axis passing from pole to pole. No definite criterion for the identi- 

 fication of the cell-axis has, however, yet been determined; for the 



