OF VITAL ACTIONS IN GENERAL. 35 



the stem of a growing tree, from the very same glutinous sap or cam- 

 bium, intervening between the wood and the bark, the wood generates, 

 in contact with its last-formed layer, a new cylinder of wood ; whilst 

 the bark produces in contact with its last-formed layer, a new cylinder 

 of bark ; the woody cylinder being characterized by the predominance 

 of ligneous fibre and ducts, and the cortical by the predommance__pf^ 

 cellular tissue. In like manner we find that, in animals, muscle pro- 

 duces muscle, bone generates bone, nerve developes nerve, in continuity 

 with itself, all at the expense of the materials supplied by the very same 

 blood. 



34. The Nutrition of tissues, by the organization of the materials 

 contained in the nutrient fluid with which they are supplied, may be 

 superficially compared, therefore, to the act of crystallization, when it 

 takes place in a mixed solution of two or more salts. If in such a solu- 

 tion we place small crystals of the salts it contains, these crystals will 

 progressively increase by their attraction for the other particles of the 

 same kind, which were previously dissolved ; each crystal attracting the 

 particles of its own salt, and exerting no influence over the rest. And 

 it is curious that if either of the crystals be broken, the new deposit 

 will take place upon it in such a mode as gradually to reproduce its 

 characteristic form. But it must be borne in mind, that such a resem- 

 blance goes no further than the surface ; for the growth of a crystal 

 cannot be really regarded as in the least analogous to that of a cell. 

 The crystal progressively increases by the deposit of particles upon its 

 exterior ; the interior undergoes no change ; and whatever may be the 

 size it ultimately attains, its properties remain precisely the same as 

 those of the original nucleus. On the other hand, the cell grows from 

 its original germ by a process of Merstitial deposit ; the substance of 

 which its wall is composed extends itself in every part ; and the new 

 matter is completely incorporated with the old. 



35. Moreover, as the increase proceeds, we see an evident distinction 

 between the cell-wall and its cavity ; and we observe that the cavity is 

 occupied by a peculiar matter, different from the substance of the cell- 

 wall, though obviously introduced through it. Of the essential differ- 

 ence which may exist in composition, between the cell-wall and the con- 

 tents of the cavity, we have a remarkable example in the case of the 

 simple Cryptogamic plant, which constitutes Yeast, and which differs in 

 no essential part of the history of its growth from the examples already 

 referred to. The principal component of its cell- walls is nearly identical 

 with ordinary cellulose f whilst the contents of the cells are closely 

 allied in composition to proteine. Again, in the fat-cells of Animals, 

 the cell-wall is formed from a proteine compound ; whilst the oily con- 

 tents agree, in the absence of nitrogen, and in their general chemical 

 relations, with the materials of the tissues of Plants. It is evident, 

 then, that one of the inherent powers of the cell, is that by which it not 

 only combines the surrounding materials into a substance adapted for 

 the extension of its wall, but that which enables it to exercise a similar 

 combining power on other materials derived from the same source, and 

 to form a compound, of an entirely different character, it may be, 

 which occupies its cavity. Now this process is as essential to our idea 



