INTRODUCTION TO FIXATION 25 



in cytology, which are seldom more than 3 mm thick, but over- 

 night fixation is not harmful. In electron-microscopy osmium 

 tetroxide is the most usual fixative. There is some evidence that 

 proteins may eventually be dissolved by this fixative, after having 

 been well fixed at first. Pieces about 1 mm thick are used, and it is 

 quite usual to fix only for an hour or two, or even for less than 

 an hour. 



The structure exhibited in a fixed microscopical preparation is 

 alm.ost always to some extent artificial, for no fixative is perfect. 

 Artifacts are of two kinds, extrinsic and intrinsic. 



Extrinsic artifacts are those formed of material brought in by 

 the fixative and deposited in the tissues. One may take as an 

 example the black granules often seen in material fixed in mer- 

 curic chloride solution (p. 36). Artifacts of this kind can usually 

 be avoided. They are commonly caused by the reaction of the 

 fixative left in the tissues with fluids in which the tissues are 

 subsequently immersed. An extrinsic artifact, once formed, may 

 often be removed by the use of special solvents. 



The intrinsic artifacts are the distorted structures of the 

 tissue-constituents themselves. Very great changes are undergone 

 by a cell when it is fixed by a coagulant fixative, embedded in 

 paraffin, sectioned, dyed, and mounted; and fixation itself is the 

 chief cause of these changes. What happens may be studied by 

 comparing fig. 4 with fig. 1 (p. 4). The cell is irregularly 

 shrunken, and the ground cytoplasm is transformed into a 

 coagulum consisting of a spongework of protein strands. No 

 part of the original substance of the cell is left in the meshes of 

 the network, which are filled only with the mounting medium. 

 The size of the meshes depends on the particular fixative used. 

 They are often even larger than those shown in the diagram 

 (fig. 4). The coarseness of the coagulum is not always realized, 

 because the strands cross one another at different depths in the 

 thickness of an ordinary section, so as not to leave any very 

 obvious gaps, and rather feeble dyes are generally used for this 

 part of the cell ; but if a crudely coagulant fixative is used, and 

 very thin sections are strongly dyed with iron haematein (p. 121), 

 the gaps become obvious. 



