CELLS. 63 



and their contents. From a few facts in vegetable physiology (growth 

 of plants in arsenical and cupreous solutions, 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 afl5rmative, and has denominated meta- 

 bolic processes of cells, all those chemical metamorphoses Avhich go on in 

 them and in their separate parts ; and justly so, for the occurrence of 

 such chemical 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-membrane, 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 diff'erent 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 resistanfwith age, and in the course of 

 ossification not only thickens, but is for the most part changed into col- 

 lagenous 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 inves- 

 tigations will be needed to show upon what it depends, whether, as it 

 would seem, the original animal cell-membrane actually alters in compo- 

 sition in course of time, or whether the change in the reaction depends 

 upon the addition of foreign substances, on the incrustation of the mem- 

 branes 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. 



The changes of the cell-contents are of two kinds, formative and re- 

 solvent. Both processes are easily followed in the embryos of different 

 animals, in which, in the first place, the primary formative cells, which 

 at the beginning are distended with the elements of the yelk, especially 

 with oil, acquire by degrees more fluid and homogeneous contents, the 

 yelk granules dissolving, sometimes from the cell-membrane towards the 

 nucleus, sometimes from within outwards ; and secondly, in cells thus 

 formed, the most various new formations take place, among which that 



