DISCOVERY OF PROTOPLASM. 



27 



complete envelope. The individual cell-cavities are often elongated and shaped like 

 either rigid or flexible tubes; or the wall of such a cavity may become very thick 

 and encroach to such an extent on the cavity that the latter is scarcely recognizable. 

 Cells of this kind look like fibres and threads, groups of them look like bundles 

 and strands, and do not resemble even remotely the cells of a honey-comb. The 

 term " cellular " is hence no longer suitable in the case of these structures. 



The expression " cellular tissue " is calculated also to occasion a wrong idea of 

 the grouping and connection of the single cell-cavities. By a tissue one would 

 surely understand a collection of thread-like elements so arranged that some of the 

 threads run parallel to one another in one direction, whilst similar threads crossing 



Tig. 6. — Cell-chambers. Showing Intercellular Spaces (i and 2) and "Intercellular Substance" (3) in the 

 Partition-walls of the Chambers. 



the first at right angles are interwoven with them. In such a tissue, as of woven 

 silk or the web of a spicier, the threads are held together by intertwining ; but this 

 is by no means the case with the collections of cells which have been called cell- 

 tissues, fiven where the parts of a so-called tissue of cells are tubular, thread-like, 

 or fibrous, they lie side by side and are joined as it were by a cement, but are never 

 crossed or twisted together like the threads in a woven fabric. 



Again, cells have been compared to the bricks of a building, but this analogy is 

 not exact. The process of formation of a cubical crystal from a solution of common 

 salt may perhaps be compared to the piling up of bricks; but when a leaf grows the 

 process is not for one layer of cells to be superimposed from the outside upon another 

 previously deposited. The development of new cells proceeds in the inside of exist- 

 ing cells and ensues from the activity of the protoplasts inclosed within the cell- 

 walls; and these protoplasts not only provide the building materials, but are them- 

 selves the builders. It is in this very fact indeed that we grasp the sole distinction 

 between organic and inorganic structures, and on this account especially the above 

 analogy is inadmissible and should be avoided. 



Cells and cell-aggregates may be conceived most clearly by considering their 

 analogy to the shells of living creatures, as we have already done more than once in 

 the foregoing pages. Protoplasts are either solitary, inhabiting isolated cell-cavities; 

 or else they live in associated groups, the cells being crowded close together in great 

 numbers and firmly attached to one another— each cavity being inhabited by one 

 such protoplast. When the latter is the case, division of labour usually takes place 



