I08 FIXATION 



it is doubtful whether chromium trioxide can be regarded un- 

 equivocally as a fixative for glycogen. 



Polysaccharides that are not naturally chromotropic are 

 rendered so by the action of chromium trioxide. This subject will 

 be considered in the chapter devoted to metachromasy (see 

 especially p. 247). 



Penetration. Chromium trioxide penetrates slowly into gelatine/ 

 albumin gel (K = i-o), and slowly also into liver (K = 0-25). 

 Tellyesniczky's data give a higher K- value for liver, but he used a 

 1% solution. It will be seen from fig. 5, a (opposite p. 67) that 

 chromium trioxide penetrates liver more slowly than picric acid. 



Shrinkage or swelling. Gelatine/albumin gels only shrink 

 slightly (to 91% of their original volume) in chromium trioxide 

 solution, and tissues in general are moderately shrunk. The 

 volume of the whole liver is reduced to 78% of the original and 

 there is further reduction to 64% in paraffin. The spermatocytes of 

 Helix, in paraffin sections mounted in Canada balsam, have 29% of 

 their original volume. In comparison with the other fixatives 

 tested by Ross, this is moderate shrinkage. 



Hardening is moderate. Wetzel's figures show that acetone and 

 ethanol leave tissues about 20 times as hard as chromium tri- 

 oxide does, while the latter leaves them about 25 times as hard as 

 they are left by 10% acetic acid. 



Immediate effects on particular constituents of the cell. As we have 

 see (p. 81), fixation by chromium trioxide is much improved by 

 the addition of an indifferent salt. It is unfortunate that in the 

 experiments of Strangeways and Canti and of Policard and his 

 colleagues, no such addition was made. Their results can be 

 briefly summarized thus. Pseudopodia, if present, are blunted ; the 

 ground cytoplasm is coagulated, sometimes rather coarsely; mito- 

 chondria are rendered invisible (and perhaps destroyed); lipid 

 droplets tend to fuse together. The nuclear membrane is rendered 

 clearly visible, the nuclear sap coarsely coagulated, the nucleolus 

 somewhat shrunken. Chromosomes are seen more clearly than in 

 life. The appearance of a cell before and after the addition of 

 chromium trioxide solution is shown in fig. 8, c, d (opposite 

 p. 70). 



Methods of washing out. It is important to get rid of the excess of 

 the fixative, lest there should be reduction at some later stage to 

 green chromic oxide, CraOg, which is insoluble in ordinary solvents 

 and remarkably resistant to acids and other reagents. Since 



