PRIMARY fixatives: NON- COAGULANTS 137 



with ethanol and has no tendency to produce insoluble extrinsic 

 artifacts, no special washing out is necessary. 



Ejfect on dyeing. Cytoplasm is rendered rather strongly acido- 

 phil, though it will also take basic dyes. The chromatin of inter- 

 phase nuclei colours rather feebly with basic dyes, and scarcely at 

 all with acid ones (probably because it is represented only by DNA, 

 the protein constituent having dissolved away). Metaphase and 

 anaphase chromosomes colour strongly with basic dyes. The 

 nucleolus is not readily coloured by dye-lakes. 



Effects on the histological picture seen in paraffin sections. Zirkle ^^^ 

 showed that the acetic anion only produced its characteristic 

 fixation image if used on the acid side of pH 4.0 or thereabouts. At 

 less acid pH than this, fixation does not occur and tissues macerate: 

 little beyond the nucleoli can be identified in paraffin sections of 

 the macerated material. 



The typical 'acid' fixation-image may be summarized thus. 

 Cell-aggregates tend to be widely separated from one another by 

 artificial spaces. Cytoplasm is poorly represented: it is strongly 

 contracted round the nuclei, or coarsely reticular. Mitochondria 

 are not seen: this is particularly characteristic of acetic fixation. 

 The shape of nuclei is fairly well retained, but the nuclear sap 

 seems not to be fixed and there is only a coarse reticulum within 

 the interphase nucleus, with a swollen, often vacuolate nucleolus. 

 Definitive chromosomes are rather well fixed. The mitotic spindle 

 appears fibrous. 



Zirkle ^^^ considered that the fixation-image was similar to that 

 given by chromium trioxide, and there are indeed similarities. 

 Nevertheless, chromium trioxide (with sodium chloride) gives 

 better general fixation. The cellular aggregates stand in more life- 

 like relation to one another; chromosomes at all stages are better 

 fixed ; the nucleolus retains its original form. 



It is not obvious why mitochondria do not appear in paraffin 

 sections fixed with acetic acid alone. Their lipid content has not 

 been proved to be soluble in the 5% solution, and indeed, as we 

 have seen, they are not necessarily destroyed by the action of the 

 fixative itself, though as a rule they are either destroyed in the 

 fixative or else left in a condition that results in their destruction at 

 a subsequent stage. In a few cases they can be seen in paraffin 

 sections of material fixed in mixtures containing a considerable 

 amount of acetic acid.^^ Despite the general belief to the contrary, 

 it must be the acetate ion or the undissociated acid that acts 



