REJUVENESCENCE IN NATURE. 155 



still lies far away from us ; yet these observations may 

 serve to make clear the importance of the cell as the 

 organ of the graduated self-determination and isolation of 

 life in all its formative movements, as the constantly 

 renewed point of separation in growth advancing by suc- 

 cessive links, and to indicate how the subordinate sphere 

 of life of the cell is the more intimately interwoven in 

 the totality of the organism, the more the specific vital 

 purpose separates into its factors. 



Examining the individual cell more closely, we must, 

 in the first place, observe that the term cell does not 

 correspond exactly to that to which we especially apply 

 it, for we understand as cells not merely the membranous 

 vesicles or utricles which form the tissue, but also their 

 contents ; we apply the name cell not alone to the little 

 chamber formed by the completely closed-in wall, within 

 which the vegetable life conceals itself, but also to its 

 living inhabitant, the more or less fluid and inwardly 

 mobile body, which is bounded, within the chamber, by 

 its own more delicate coat (the primordial utricle). The 

 cell is thus a little organism, which forms its covering 

 outside, as the muscle, the snail, or the crab does its 

 shell. The contents enclosed by these envelopes form 

 the essential and original part of cell, in fact must be 

 regarded as a cell, before the covering is acquired. From 

 the contents issues all the physiological activity of the 

 cell, while the membrane is a product deposited outside, 

 a secreted structure, which only passively shares the life, 

 forming the medium of intercourse between the interior 

 and the external world, at once separating and combining 

 the neighbouring cells, aflbrding protection and solidity 

 to the individual cell in connection with the entire tissue. 

 Hence the development of the cell-coat, as a product of 

 cellular activity, always stands in inverse proportion to 

 the physiological activity of the cell. In youth, thin, 

 soft, and extensible, the cell-coat allows abundant 

 nutrition and advancing growth ; subsequently thickened 

 and therewith hardened by the deposit of lamellae, it 



