CELLS. 39 



tions, without the admission of these substances), it might be 

 believed that the membranes exercised the more important 

 influence in determining absorption. 



It is an important question, whether the substances received 

 by the cells and composing them become modified by their 

 vital processes. Schwann has answered it in the affirmative, 

 and has denominated metabolic processes of cells, all those 

 chemical metamorphoses which go on in them and in their 

 separate parts; and justly so, for the occurrence of such che- 

 mical changes is not only very probable a priori, since in plants 

 all such metamorphoses (and these of the most various kinds) 

 take place in the cells, but may very easily be demonstrated 

 by observation. These changes affect, firstly, the cell-mem- 

 brane, and secondly the cell-contents. As regards the former, 

 this much is certain, that the membranes of most cells not only 

 become denser and more solid with age, but also that they 

 take on a different chemical constitution, though it is impossible 

 in particular cases to say on what the change depends. In the 

 horny tissues, the membranes of the young cells are easily 

 soluble in alkalies and acids, whilst subsequently they sometimes 

 offer extreme resistance to their action : the same takes place 

 in a few of the higher elementary parts, as the nervous tubules, 

 the animal muscles, and the capillaries, in which the sarcolemma, 

 the sheath of the nerve fibre, the capillary membranes, which 

 are metamorphosed cell-membranes, react in a very different 

 manner from the original formative cells. In the cartilage 

 cells also, the membrane becomes more resistant with age, and 

 in the course of ossification not only thickens, but is for the 

 most part changed into collagenous tissue, which is subsequently 

 impregnated with calcareous salts. These examples, which 

 might be multiplied, may suffice to demonstrate the occurrence 

 of a metamorphosis of the cell-membranes; further investi- 

 gations will be needed to show upon what it depends, whether, 

 as it would seem, the original animal cell-membrane actually 

 alters in composition in course of time, or whether the change 

 in the reaction depends upon the addition of foreign substances, 

 on the incrustation of the membranes with salts, and so forth, 

 such as botanists are inclined to assume for the vegetable cell- 

 membranes, or whether it depends upon secondary deposits on 

 the exterior of the original membranes. 



