I06 FIXATION 



it unlikely that the potential would differ significantly from this. 

 Chromium trioxide is a much stronger oxidizer than any other 

 fixative. 



The oxidation-potential does not change measurably even 

 during prolonged fixation. ^^^ 



Manufacture. Chromium trioxide is prepared by mixing excess 

 of concentrated sulphuric acid with a saturated aqueous solution 

 of potassium or sodium dichromate. 



Introduction as fixative. Chromium trioxide was introduced into 

 microtechnique by Hannover in 1840. He had been staying in 

 Copenhagen with a Professor Jacobson, who had experimented 

 with the use of this substance in medicine. On the day of Han- 

 nover's departure, Jacobson had shown him a divided mammalian 

 eye, preserved in a solution of chromium trioxide. Hannover was 

 struck by the state of preservation of the organ, and on his return 

 home he tried hardening various tissues in 5% or even stronger 

 solutions and then cutting sections for microscopical study. He 

 found that the consistency was good for cutting. He mentions a 

 large number of tissue- constituents that were well preserved, 

 including blood-corpuscles, cartilage-cells, medullated nerve- 

 fibres, and ciliated epithelium from the mouth of the frog. His 

 paper ^^^ takes the form of a letter to Jacobson. 



Chromium trioxide was used by Corti ^^^ in 1851 for the fixation 

 of the inner ear, and by Miiller ^^^ in i860, in a mixture with 

 potassium dichromate and sodium sulphate, to fix an abnormal 

 human eye. The discovery that it is useful in the fixation of chro- 

 mosomes was made by the Polish cytologist Mayzel ^^^ in 1878. 

 Strasburger *^^ in 1879 found a 1% solution the best fluid for 

 maintaining plant chromosomes in their living form, and he made 

 much use of it in his subsequent work. Flemming exploited 

 Mayzel's discovery from 1879 onwards. ^'^' ^^^ Much of his 

 pioneer work on chromosomes was done with chromium trioxide 

 at o-i to 0-2%. 



Reactions with proteins. This is a powerful coagulant of 

 albumin, but it does not stabilize gelatine gels. It is exceptional 

 among fixatives in rendering proteins wholly resistant to digestion 

 by pepsin and trypsin. Chromium cannot be removed from tissues 

 even by prolonged washing, and it is probable that this is an 

 additive fixative; but very little is known about the chemical 

 changes that take place when solutions of chromium trioxide react 

 with proteins at room-temperature. Tyrosine, trytophane, and 



