GENEEAL BIOLOGY. 7 



Though many points are still in dispute, it may be safely 

 said that the nucleus plays, in most cells, a r6le of the highest 

 importance ; in fact, it seems as though we might regard the 

 nucleus as the directive brain, so to speak, of the individual 

 cell. It frequently happens that the behavior ef the body of 

 the ceU is foreshadowed by that of the nucleus. Thus fre- 

 quently, if not always, division of the body of the nucleus pre- 

 cedes that of the cell itself, and is of a most complicated char- 

 acter (karydkinesis or mitosis). The cell wall is of subordinate 

 importance in the processes of life, though of great value as a 

 mechanical support to the protoplasm of the cell and the aggre- 

 gations of cells known as tissues. The greater part of a tree 

 may he said to be made up of the thickened walls of the cells, 

 and these are destitute of true vitality, unless of the lowest 

 order; while the really active, growing part of an old and large 

 tree constitutes but a small and limited zone, as may be learned 

 from the plates of a work on modern botany representing sec- 

 tions of the wood. 



Animals, too, have their rigid parts, in the adult state espe- 

 cially, resulting from the thickening of a part of the whole of 

 the cell by a deposition usually of salts of lime, as in the case of 

 the bones of animals. But in some cases, as in cartilage, the 

 cell wall or capsule undergoes thickening and consolidation, 

 and several may fuse together, constituting a matrix, which is 

 also made up in part, possibly, of a secretion from the cell pro- 

 toplasm. In the outer parts of the body of animals we have a 

 great abundance of examples of thickening and hardening of 

 cells. Very well-known instances are the indurated patches 

 of skin (epithelium) on the palms of the hands and else- 

 where. 



It will be scarcely necessary to remark that in cells thus 

 altered the mechanical has largely taken the place of the vital 

 in function. This at once harmonizes with and explains what is 

 a matter of common observation, that old animals are less act- 

 ive — ^have less of life within them, in a word, than the young. 

 Chemically, the cellulose wall of plant-cells consists of carbon, 

 hydrogen, and oxygen, in the same relative proportion as exists 

 in starch, though its properties are very different from those of 

 that substance. 



Turning to cell contents, we find them everywhere made up 

 of a clear, viscid substance, containing almost always granules 

 of varying but very minute size, and differing in consistence 



