6 THE CELL [CH. 



another. But it must always be remembered that the pro- 

 cess of fixation, and possibly also the subsequent embedding, 

 section-cutting, mounting, and staining, may produce im- 

 portant artificial modifications in the original structure, or 

 even give rise to structure which did not exist in the living 

 cell. A comparison of the appearance produced by different 

 fixatives, and of the fixed with the living cell, shows that 

 in general the coarser and more obvious structures are 

 probably in most cases faithfully preserved by suitable 

 fixatives, but when one comes to the finer details it is ex- 

 tremely difficult to be sure that they in any way represent 

 the true structure as it existed in life. In the living cell all 

 the parts are commonly colourless and transparent, and 

 are therefore only distinguishable by differences in refran- 

 gibility, and such differences are visible only in the case of 

 bodies of a certain size. In the sections of the fixed and 

 stained cell many much finer details are easily seen, but in 

 interpreting these attention must always be paid to the 

 possibility that they have undergone more or less serious 

 modification in the process of preservation, or even that 

 they are purely artificial. That this danger is not an imagin- 

 ary one is easily realised by comparing preparations of the 

 same tissue fixed and stained by different methods, or even 

 by noting the variations in different parts of a single pre- 

 paration, in cases in which the fixative is not very pene- 

 trating, and in which the outer portions may have a quite 

 different appearance from those at a greater depth from 

 the surface. Even when the various methods used all result 

 in what are called "well-fixed" preparations, quite con- 

 siderable differences may follow the different proceedings, 

 and in "badly-fixed" tissues the existence of purely artificial 

 structures is often perfectly evident. In practice, however, 

 the difficulty of determining whether a structure is natural 



