PRACTICAL FIXATIVE SOLUTIONS 57 



the tissue ahead of the fixative, and prevents the initial swelling 

 and bursting.^®' Certain difficulties in accepting this hypothesis as 

 a complete explanation have been discussed rather fully else- 

 where. ^^ 



Salts used for the purpose just mentioned may play another 

 role as well. Phospholipids have a tendency to alter their form by 

 sending out microscopical, tentacle-like 'myelin forms' into sur- 

 rounding water. Calcium salts prevent this change. ^°® In studies 

 of the lipid constituents of cells it is therefore reasonable to sub- 

 stitute calcium chloride for sodium chloride in fixative solutions. ^* 

 The concentration must be made slightly higher than that of 

 sodium chloride, if the same osmotic pressure is to be obtained. 

 The following formula is suitable : 



Distilled water . . . . . 80 ml 

 Calcium chloride (anh.), 10% aq. . . 10 ml 

 Formalin . . . . . . 10 ml 



Keep marble in the fluid. pH (in the presence of marble), 5-7. Wash 

 out with water or 50° o ethanol. 



Formaldehyde is much used in these simple solutions in histo- 

 chemical studies, especially of lipids. To make sure that phospho- 

 lipids will be fixed, the tissue may be 'postchromed'. After quite 

 short fixation (6 hours) in the formaldehyde solution, it is trans- 

 ferred (without washing) to a solution of potassium dichromate. 

 A solution of this salt maintained at saturation in an incubator at 

 37^ C is suitable. Treatment for a day or two suffices. The tissue 

 must then be washed in running water overnight. More elaborate 

 methods of postchroming are also available. ^^' ^® 



OSMIUM TETROXIDE IN SIMPLE FIXATIVES 



Osmium tetroxide gives excellent results when unmixed with 

 other primary fixatives, provided that a suitable embedding 

 medium, such as methacrylate (p. 77), is used. Gross shrinkage 

 and distortion occur if tissues fixed in this way are embedded in 

 paraffin.-^ There is evidence^^^ that the submicroscopical struc- 

 ture of cells is best stabilized if the fixative solution is buff'e red just 



