104 FIXATION 



ethanol. The precursor of the crystals is also removed by this 

 treatment. 



The removal of mercury deposits may be delayed until sections 

 have been prepared, but it is best to treat the tissue in bulk w^ith 

 iodine solution, as this makes it less brittle and therefore easier to 

 cut. When sections have been prepared but not yet dyed, a glance 

 through the microscope will show w^hether a second treatment with 

 iodine is necessary. 



Since protein coagulates may, in certain circumstances, be 

 soluble in iodide solutions (p. 54), it is best not to dissolve the 

 iodine with potassium iodide. 



The colour of iodine is removable by soaking in 70% ethanol, or 

 almost instantly by the action of sodium thiosulphate. This method 



2[S203]= + I2 = [S40e]= + 2I- 



thiosulphate tetra- iodide 



thionate 



of removing iodine was introduced into microtechnique by 

 Heidenhain (1909). 



Effect of dyeing. Within the range of pH at which it is ordinarily 

 used in fixation, mercuric chloride leaves the tissues more re- 

 ceptive of dyes in general than any other fixative. Cytoplasm will 

 accept both basic and acid dyes, particularly the former. Chromatin 

 is rendered very strongly colourable by basic dyes and dye-lakes. 



Effects on the histological picture seen in paraffin sections. Mercuric 

 chloride by itself gives grade IV fixation. Cell- aggregates are not 

 seriously distorted. Cytoplasm is rather homogeneously fixed, but 

 it is badly shrunken and there is a tendency for cells to separate 

 from one another. Red blood-corpuscles are not badly fixed; 

 haemoglobin is retained within them (perhaps as methaemoglo- 

 bin *^^). The nuclear membrane is clearly shown; a nuclear net- 

 work is not produced; chromosomes are not well fixed. 



Compatibility with other fixatives. Mercuric chloride may be 

 mixed with any other fixative. In certain mixtures, acetic acid 

 appears to reduce its solubility. ^^* 



CHROMIUM TRIOXIDE 



Standard concentration for fixation. 0-5% w/v aqueous solution. 

 Formula and formula-weight. CrOg. loo-o. 

 Description. Chromium trioxide consists of brownish-red crystals, 

 readily deliquescent by absorption of water. Their melting-point 



