STRUCTURE OF LIVING THINGS 



175 



shown that the box-like case or cell-wall (the original 

 " cell " of Hook) is actually formed by the living 

 nucleated plasm or viscid matter within it, just as a snail 

 forms its shell, by the 

 separation or "secre- 

 tion " of a dead, firm, 

 chemical deposit on 

 its living surface. 

 Schwann showed that 

 a 1 1 n o t merely 

 special exceptional 

 instances, but all 

 the tissues of plants 

 and of animals are 

 built up by nucleated 

 cells, the cell-wall be- 

 ing often not hard 

 and box-like, but soft, 

 gelatinous, irregular 

 in shape, and some- 



FlG. 40. Three kinds of cells, magnified a 

 thousand times linear. A, a row of cilia- 

 bearing cells. B, a single detached ciliated 

 cell : observe the nucleus in each cell. C, 

 a goblet-cell, from a mucous surface, pro- 

 ducing f, a slimy secretion ; d, the wall of 

 the cell ; b, the nucleus ; a, the protoplasm 

 in which the secretion c was accumulated 

 until it burst out at the free end of the cell. 

 D, a fat-cell ; a, the nucleus surrounded by 

 protoplasm ; e, the thin layer of protoplasm 

 enveloping the great oil drop /, which has 

 formed within it. 



times very thin, 

 sometimes very thick. 

 Every living cell is 

 thus surrounded by 

 the chemical products 

 of its own activity, or 

 may deposit those 

 products within itself 

 as in the goblet-cell 

 and the fat- cell seen 



in Fig. 40, C and D, 



and these products differ in different tissues. The cells 

 of a tissue, using the word to mean the soft nucleated 

 particles or corpuscles of protoplasm or " cell-substance," 

 must be regarded as the microscopic living "weavers" 



