CYTOLOGY 33 



almost always enclosed in a cell-wall of definite shape. 

 Individual cells are very seldom large enough to be seen 

 with the naked eye, though egg cells are in some families 

 large enough to be recognised, and in some cases fibres 

 and hairs several millimetres or more in length are com- 

 posed of a single cell. In general, however, the study 

 even of the grosser features of cells can only be under- 

 taken through the microscope. To see the finer details 

 an exceedingly high power of magnification is required. 

 To separate an individual living cell from the rest in a 

 tissue is not easy, and yet for examination under high 

 magnification the specimen must be exceedingly thin ; 

 even two of the smallest cells lying on the top of 

 each other are too opaque for microscopic examination, 

 consequently mechanical means are employed to cut 

 thin sections of the tissues. The material to be ex- 

 amined is killed and " fixed " by some chemical solu- 

 tion which quickly penetrates to the finest ultimate 

 structures in the cells, so that they remain as nearly 

 as possible exactly as they were when alive. Many 

 hundreds of sections may be cut from an object that 

 is being studied, and the course of the life processes is 

 reconstructed from them. Thus it happens that the 

 motions and behaviour of the nuclei, for instance, 

 though described as if from observations made on a 

 living specimen, are seldom based on actual observa- 

 tions, and our knowledge of them is reconstructed 

 from innumerable fixed sections. 



The first glance at a parenchyma cell shows that the 

 mass of protoplasm within its wall is finely granular, 

 and that in it there is a darker mass, also granular, 

 which is often found in a somewhat central position, 

 and is called the nucleus. The nucleus is the most 

 vital part of the cell, and its elaborate behaviour has 



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