CELLULAR DEVELOPMENT. 151 



considerations concerning generation until we shall trace the 

 union of the sperm and germ cells, and are prepared to study 

 the development resulting therefrom. We can the better 

 proceed in this course, if it is continually borne in mind that 

 the cell is in a certain sense an independent being, and rules 

 arbitrarily over certain surrounding limits. In some tissues, 

 where there are intercellular substances, each cell rules, as 

 pathological investigation shows, over its own defined terri- 

 tory. In other tissues in which the cells are contiguous, each 

 cell can run its own course without the fate of the cell lying 

 next to it being necessarily linked with its own. In a third 

 tissue we find the cell-elements more intimately connected 

 with each other : as, for instance, a stellate-cell may anasto- 

 mose with a similar one, and in this way a reticular arrange- 

 ment may be produced similar to that seen in capillary-vessels 

 and other analogous structures. Yet even in this chain-work 

 of cells, individual cells, in consequence of certain internal 

 or external influences, undergo certain changes confined to 

 their own limits, and not necessarily participated in by cells 

 adjoining. ( Virchow. ) 



Each cell in its development reproduces itself. — The manner 

 of the development need not concern us in this place. 

 Although primarily the ideal cells are in form alike, yet 

 through the differentiation arising from complexity of struc- 

 ture, we see existing cells of apparently widely different 

 origin. We have, for instance, the hepatic-cell , columnar 

 epithelium, cells of connective tissue, muscle-cells, nerve- 

 cells, etc., etc. In the animal organism there is a continual 

 using up of tissues. The food passing into the body supplies 

 the material for supplying the wastes, and cells absorbing 

 their share at the necessary time reproduce or repair their 

 form or their substance. That form of epidermic cells found 

 in the nail produces nail-cells ; epithelial-cells, epithelium, etc. 

 When the hand is cut the muscles unite by their own appro- 

 priate tissue ; the skin heals in a like manner. When the 

 nail is injured the remaining cells multiply themselves, as in 

 growth, until the injury is more or less repaired. Remove 

 the nail-cells, and the surrounding cells are unable to develop 

 themselves into nail. 



Each cell in its development is affected by its environment. — If 



