NON-COAGULANT PRIMARY FIXATIVES 53 



reason for its use as a component of fixative mixtures. Its action 

 offsets the shrinkage caused by other substances used in micro- 

 technique. Gelatine-albumin gel expands to between 4 and 5 times 

 its original volume in 18 hours (fig. 5, p. 27). Acetic acid has much 

 more swelling effect than the mineral acids have. This is partly 

 because the pH is not low enough to cause coagulation; partly, it 

 seems, because the undissociated acid is responsible for some of 

 the swelling. Thus, acetic acid has a swelling (or anti-shrinking) 

 effect even when it is used as a constituent of a non-aqueous 

 mixture, such as Clarke (see p. 59). In aqueous media acids are 

 thought to break the salt-links that connect protein chains; the 

 hydrophil groups thus exposed would draw water into the gel or 

 tissue. 



Hardening. Leaves tissues much softer than any other fixative 

 does. 



Method of washing out. Since acetic acid is miscible with 

 ethanol in all proportions and has no tendency to produce 

 extrinsic artifacts, no special washing out is necessary. 



Effect on the appearance of ceils in microscopical preparations. 

 The external form of the cell is fairly well preserved. So, in general, 

 are the cytoplasmic inclusions, but the mitochondria become less 

 visible and may disappear. The nuclear contents are transformed 

 into a coarse network. 



In parafhn sections, cell-aggregates tend to be widely separated, 

 with artificial spaces in between. Cytoplasm is sometimes coarsely 

 reticular, sometimes contracted round the nuclei. Mitochondria 

 are absent. The nuclear sap is coarsely reticular; the nucleolus is 

 swollen; metaphase and anaphase chromosomes are rather well 

 fixed; the mitotic spindle appears fibrous. 



The acetate ion only produces its characteristic fixation-image 

 on the more acid side of pH 4 or thereabouts. ^^° On the less acid 

 side there is no fixation and the tissues macerate. 



Cytoplasm takes acid dyes strongly, basic ones rather feebly. 

 The chromatin of interphase nuclei colours feebly with basic dyes, 

 scarcely at all with acid ones. Metaphase and anaphase chromo- 

 somes colour strongly with basic dyes. 



Compatibility with other fixatives. Compatible with all other 



