38 CYTOLOGICAL TECHNIQUE 



Reactions with nucleic acids. DNA is precipitated from solution 

 in an insoluble form/^ 



Reactions with lipids. The fat of adipose tissue can be made 

 insoluble in lipid-solvents by prolonged treatment with chromium 

 trioxide/^*' ^^ but the ordinary period of fixation does not suffice. 

 The reactions with lipids have not been studied in detail, but they 

 probably resemble those of potassium dichromate (p. 50). 



Reactions with carbohydrates. Polysaccharides are converted to 

 aldehydes, by oxidation, and will then respond to colour-tests for 

 aldehydes. ^^ It is not certain that chromium trioxide actually fixes 

 glycogen, but the aldehyde formed seems less liable to be dissolved 

 out of the tissues by water than glycogen itself is. 



Rate of penetration. Slow {K = 1 0). 



Shrinkage or swelling. Shrinks gelatine-albumin gel slightly, 

 tissues considerably. 



Hardening. Moderate. 



Method of washing out. It is usual to wash out in running 

 water. If any free chromium trioxide is left in the tissues, it may 

 subsequently be reduced to green chromic oxide, Cr^Oa, by 

 ethanol or some other reducer. The oxide is difficult to remove 

 from the tissues, as it is insoluble in ordinary solvents and 

 resistant to acids and other reagents. 



Effect on the appearance of cells in microscopical preparations. 

 Ground cytoplasm is rather coarsely coagulated and there may be 

 some distortion of the shape of the cell ; the nucleus is on the 

 whole well preserved, the chromosomes excellently. 



In paraffin sections, cell-aggregates are quite well fixed; so is 

 the nucleus, though the nuclear sap is changed to a coarse 

 coagulum; the chromosomes and nucleolus are particularly well 

 shown; mitochondria and lipid droplets are not seen. 



Cytoplasm has a strong affinity for acid dyes and is therefore 

 easily dyed in a diff"erent colour from the nucleoprotein, though 

 the latter has not such a strong affinity for basic dyes as it has 

 after fixation by mercuric chloride or formaldehyde. 



Compatibility with other fixatives. It can be mixed with mer- 

 curic chloride, osmium tetroxide, or acetic acid. If it is mixed with 

 potassium dichromate, the special properties of the latter sub- 



