PRIMARY fixatives: NON-COAGULANTS 13I 



Reactions with carbohydrates. Potassium dichromate is not a 

 fixative for glycogen. Chromium is not known to be taken up from 

 solutions of potassium dichromate by any carbohydrate or related 

 substance, with the possible exception of lignin.^^ It is to be pre- 

 sumed, however, that acidified potassium dichromate will react 

 towards carbohydrates in the same way as chromium trioxide 

 (p. 107). 



Penetration. Telly esniczky's data give a high K- value (i'33) for 

 the penetration of the 3% solution into liver, but it is doubtful 

 whether this means much. The term rate of penetration, as used in 

 this book, means the rate of penetration with fixative effect. Now 

 potassium dichromate does not coagulate proteins, nor does it 

 gelatinize most of them in the ordinary period of fixation. Figures 

 for rate of penetration do not seem to be applicable to this sub- 

 stance, though no doubt it runs quickly through protein gels ^^^ 

 and tissues. 



Shrinkage or swelling. Gelatine/albumin gels swell strongly (to 

 160% of their original volume) in potassium dichromate solution. 

 Whole livers remain unchanged in volume in a 3% solution, but 

 are shrunken by subsequent dehydration and retain only 49% of 

 their original volume when brought into paraffin wax. Primary 

 spermatocytes of the snail are reduced to 23% of their original 

 volume when paraffin sections of the ovotestis fixed in a 5 % solu- 

 tion have been mounted in Canada balsam. This represents greater 

 final shrinkage than that which follows fixation by most primary 

 fixatives. 



Hardening. Tissues are left very soft. Wetzel's figure for rigidity 

 after fixation in a 3% solution is 171. Chromium trioxide leaves 

 tissues 2*7 times as rigid as this. After subsequent soaking in 80% 

 ethanol tissues are still very soft. 



In the old days, when potassium dichromate was used as a 

 hardening agent before sectioning by hand without embedding, 

 tissues were left in the solution for long periods. 



Immediate effects on particular constituents of the cell. The shape 

 of the cell is rather well preserved, though there may be some 

 retraction of small pseudopodia. The ground cytoplasm becomes 

 somewhat granular. Mitochondria are preserved, but transformed 

 from threads into ovoids and short rods: their form would prob- 

 ably be better maintained in the presence of an indifferent salt. 

 Lipid globules tend to run together. The nucleus retains its form- 

 but may be somewhat retracted away from the cytoplasm; its 



