PRIMARY fixatives: COAGULANTS IO9 



ethanol might cause this reduction, tissues are usually washed in 

 running water before dehydration. Virchow ^^^ showed that no 

 insoluble precipitate was formed on direct transference of tissues 

 to 95% ethanol, if light was excluded. Overton ^"'^ recommended 

 the use of sulphurous acid after fixation. He thought that chromic 

 sulphate was formed: this, being stable and soluble, would be 

 harmless, and indeed would act as a mordant for certain dyes. 

 Direct transference from solutions of chromium trioxide to 50% 

 ethanol containing 2% of sulphuric acid is probably safe.^^ 



Effect on dyeing. Cytoplasm is rendered strongly acidophil by 

 chromium trioxide, while chromatin is fairly easily colourable by 

 most basic dyes. Differential dyeing of cytoplasm and chromatin is 

 therefore easy, though there is some tendency for acid dyes to 

 colour chromatin. 



Effects on the general histological picture seen in parajfin sections. 

 The excellence of chromium trioxide for the fixation of the 

 neurones of Sepia, when dissolved at i % in sea-water, was noted 

 by Young. ^^^ In the presence of an indifferent salt at an appro- 

 priate concentration, this substance gives a better general picture 

 than any other primary fixative, with the possible exception of 

 chloroplatinic acid.^^ These two fixatives, with sodium chloride at 

 0*75%, fall into grade II. This grade is not reached by any 

 other primary fixative than these (though it could be reached 

 easily enough if paraffin embedding were not employed). In the 

 absence of sodium chloride, chromium trioxide falls into grade III. 



The general picture given by chromium trioxide is as follows. 

 Cellular aggregates are fixed without serious distortion; cyto- 

 plasm is sometimes contracted round the nucleus, or coarsely 

 coagulated so as to leave wide meshes; mitochondria are not 

 fixed; red blood-corpuscles are swollen. The form of the nucleus 

 is well preserved; the nuclear membrane is clearly seen; the 

 nuclear sap is rather coarsely coagulated ; the nucleolus is well fixed 

 and has a strong affinity for both iron haematein and acid fuch- 

 sine; chromosomes are very well shown. The mitotic spindle 

 appears fibrous. 



The concentration at which this fixative is used makes remark- 

 ably little difference. Even at o-oi% (with sodium chloride at 

 0*^5%)' chromium trioxide gives much the same picture as when 

 used at the ordinary concentrations. ^^^ 



Compatibility with other fixatives . It is best not to mix chromium 

 trioxide with substances that will reduce it, such as formaldehvde 



