PRIMARY fixatives: NON-COAGULANTS 133 



a halo, and may be subdivided; the heterochromatic segments of 

 the chromosomes are not seen in the interphase nucleus, and the 

 definitive chromosomes of mitosis and meiosis are unfixed. 



In brief summary and at some risk of over-simplification one 

 may express the effect of pH on the action of potassium dichromate 

 thus. On the less acid side of the critical range (pH 3-4 to 3-8), the 

 cytoplasm, nuclear sap, and mitotic spindle are homogeneously 

 fixed ; mitochondria are retained ; the nucleolus is partly dissolved ; 

 chromosomes are scarcely visible. On the more acid side potassium 

 dichromate acts like chromium trioxide: that is to say, cytoplasm 

 and nuclear sap are coarsely coagulated and the mitotic spindle is 

 fibrous; mitochondria are non-existent; the nucleolus and chromo- 

 somes are well fixed. These may be called respectively the less 

 acid and more acid fixation-images of the chrome anions, as seen 

 in paraffin sections. 



Compatibility uitli other fixatives. Potassium dichromate is 

 compatible with picric acid, mercuric chloride, and osmium 

 tetroxide. If mixed with more than a very small amount of 

 chromium trioxide, it shows the more acid fixation-image. ^^^' ^^^ 

 It also shows this image if mixed with more than a very small 

 amount of acetic acid or any other acid used in fixation. It reacts 

 rather slowly with formaldehyde, and mixture with this substance 

 is allowable if it is done immediately before use ; when the colour 

 changes, the fluid should be renewed. Potassium dichromate 

 solution should not be mixed with ethanol, lest chromic oxide be 

 deposited in the tissue. 



Uticlassified remarks. It has been known since the end of the last 

 century that different dichromates give different fixation-images,^^ 

 but the explanation awaited the work of Zirkle.^^^ The subject has 

 now been re-investigated by Casselman. ^^^ Those dichromates 

 that show, at fixative concentration, a pH on the more acid side of 

 the critical range, fix like chromium trioxide: barium, calcium, 

 mercuric, and silver dichromates are examples. A saturated solu- 

 tion of mercuric dichromate is particularly strongly acid (pH 

 1-05). Those that show a pH on the less acid side fix like potassium 

 dichromate. The ammonium, lithium, and sodium salts do this. 

 The last-named is the least acid, a solution of the same mole- 

 cular concentration as 2-5% potassium dichromate (pH 4*05) 

 showing a pH of 5-10.^^^ Ammonium dichromate presents the 

 advantage that it does not swell mitochondria, as the potassium 

 salt does.^^^ 



