PRIMARY fixatives: NON - CO AGUL ANTS I23 



unsaturated, it is not clear why they remain colourless. Perhaps 

 they tend to be simply oxidized instead of forming additive com- 

 pounds. 



Since osmium tetroxide is soluble in certain lipids, it can be 

 taken up without change by fully saturated ones, and then reduced 

 by subsequent soaking of the tissue in ethanol (compare Starke ^^^). 

 Thus the fact that a lipid is black in a paraffin section does not 

 prove that it is unsaturated. 



Reactions with carbohydrates. It appears that osmium tetroxide 

 does not react with most hexoses or pentoses or their polymers at 

 room-temperature, though sucrose is very slowly oxidized to 

 oxalic acid.^^ There is some darkening if glycogen is treated with 

 osmium tetroxide for long periods at 50° C.^^ 



Rate of penetration. Osmium tetroxide penetrates slowly into 

 gelatine/albumin gel. The K-value for 25 hours is 0-85, which is 

 nearly as low as the figure for picric acid. The K-value gradually 

 falls off with time. During the first 16 hours it is i-o, but during 

 the period 16 to 144 hours it is only 0-31. It must be supposed that 

 the osmium deposited in the gel (whether in combination with 

 protein or in the form of the dioxide) presents an obstacle to 

 diffusion. A measurable fall-off in K-value is not known to occur 

 with any other fixative. 



Tellyesniczky's data ^^^ for penetration into liver (12 hours) give 

 K- values of 0-29 for the 0-5% solution and 0-58 for the 2%. This 

 indicates that the i % solution would be one of the more slowly 

 penetrating fixatives. 



Shrinkage or swelling. Gelatine/albumin gel shrinks very slightly 

 (by less than 10% of its volume) in osmium tetroxide solution. 

 The change of volume of whole livers has not been measured, 

 presumably because the experiment would be too expensive. It is 

 unfortunate that there are no satisfactory numerical data for the 

 shrinkage or swelling of cells. Kaiserling and Germer ^^^ found 

 that mammalian eggs increased somewhat in diameter when trans- 

 ferred from saline to osmium tetroxide dissolved in distilled water. 

 The saline was hypotonic and the eggs had already swollen some- 

 what in it. 



Hardening. Osmium tetroxide leaves tissues rather soft. Wetzel's 

 figure is 171 ; the figure for chromium trioxide is 1-4 times greater. 

 Tissues fixed in osmium tetroxide are crumbly in paraffin and do 

 not section well. 



Immediate effects on particular constituents of the cell. By common 



