NON-COAGULANT PRIMARY FIXATIVES 51 



lecithin) are able to take up the metal. ^° It seems probable that 

 chromium attaches itself to the phosphate group, though other 

 suggestions have been made.^^^ 



The possibility has been mentioned above (p. 50) that potas- 

 sium dichromate may fix ground cytoplasm by acting on the lipid 

 component. 



Reactions with carbohydrates. The unacidified salt is not known 

 to fix any carbohydrate. If acidified, it presumably acts like 

 chromium trioxide (p. 38). 



Rate of penetration. Since proteins are neither coagulated nor 

 gelled in the ordinary period of fixation, the rate of penetration 

 cannot be measured by the method adopted with other fixatives. 

 There is no satisfactory information about the rate of penetration 

 of this substance. 



Shrinkage or swelling. Gelatine-albumin gel is swollen by potas- 

 sium dichromate. Whole livers remain unchanged in volume in a 

 3 % solution, but they are subject to severe subsequent shrinkage 

 by ethanol. 



Hardening. Tissues are left very soft. 



Method of washing out. To avoid the possibility that insoluble 

 chromic oxide (CroOg) will be precipitated in the tissues through 

 subsequent reduction by ethanol, it is usual to wash out in run- 

 ning water. This reduction does not occur, however, if light be 

 excluded. ^^^ 



Effect on the appearance of cells in microscopical preparations. 

 The cell maintains its form rather well. Mitochondria are pre- 

 served, but filamentous ones may be changed into ovoids. The 

 nuclear sap becomes finely granular. The nucleolus shrinks (pre- 

 sumably through loss of RNA). 



In paraffin sections there is a shrinkage apart of cell aggregates, 

 with production of artificial spaces ; the cytoplasm, though rather 

 homogeneous, is shrunk round the nucleus; mitochondria are 

 retained, but swollen ; mitotic and meiotic chromosomes and the 

 heterochromatic segments of the interphase nucleus are unfixed. 

 If the fixative solution is acidified below pH 3-4, the appearance 

 is the same as that given by chromium trioxide. 



Both basic and acid dyes act quite strongly. The chromatin, 



