128 FIXATION 



The fact that potassium dichromate is unsuitable for use in 

 studies of mitosis was first made known by Mayzel.^^^ 



Reactions with proteins. This is a non-coagulant of albumin 

 solution, but it very gradually renders undiluted egg-white more 

 viscous and eventually transforms it into a weak, semi-transparent 

 gel. 



Since, as we have seen (p. 126), the chrome anions are almost 

 the same whether potassium dichromate or chromium trioxide be 

 dissolved, it must be supposed that the striking differences be- 

 tween the effects of the two substances on proteins must be due to 

 the large difference in pH. If potassium dichromate be acidified, it 

 reacts with proteins like chromium trioxide: that is to say, it 

 becomes a strongly coagulant fixative. The change-over from one 

 behaviour to the other occurs in the pH-range 3-4 to 3*8.^^^ As 

 Casselman points out,^^^ this is near to the iso-electric points of 

 many proteins (though somewhat below that of most). It must be 

 supposed that in the region of the iso-electric point, the proteins 

 change radically in their reactions to the chrome anions. The 

 chemical changes concerned in the slow gel-forming process that 

 occurs above the critical pH-range have not been investigated. 



If gelatine that has been impregnated with potassium dichromate 

 is exposed to light, it becomes insoluble in warm water. This fact is 

 used in the *Autotype' process of photographic printing. It is 

 interesting to notice that chromium trioxide, on the contrary, 

 makes protoplasm soluble on exposure to bright light (p. no). In 

 the ordinary circumstances of fixation, gelatine/albumin gel is not 

 stabilized against warm water by the action of potassium 

 dichromate. 



Nucleoprotein solution is not coagulated by potassium dichro- 

 mate, and DNA is readily dissolved by this salt; the histone of the 

 nucleus, however, is strongly gelatinized by potassium dichro- 

 mate. ^^^ There is a marked contrast here with the effect of acetic 

 acid, which precipitates DNA but dissolves histone (see p. 135). 



Reactions zvith lipids. Potassium dichromate is able to attach 

 chromium to certain lipids, and to render them insoluble in lipid 

 solvents. The chromium can subsequently be made to react with 

 haematein to give a black lake. This is the essence of Weigert's ^^^ 

 method for myelin. Tissues were fixed in Miiller's fluid, embedded 

 in collodion, and treated with a solution of haematein (often called 

 Weigert's 'haematoxylin', but haematoxylin is not a dye (see 

 p. 173)). Myelin was coloured black. Other tissue-constituents 



