FIXATIVE MIXTURES 147 



from the formaldehyde till the last moment, but there is no reason 

 why the formaldehyde should not be mixed with the acetic acid. 

 Although certain very unstable fixatives w^ork well, yet it is 

 questionable whether they could not be replaced by more rational 

 mixtures. Some of the changes that occur in unstable fixatives have 

 been studied by Freeman and his colleagues ^"^ and by Cassel- 

 man.^^^ The fluids of Helly, Karpechenko, Orth, and Regaud all 



TABLE 10 



Method for making up small quantities of certain cytologic al fixatiz'es. 

 It is often convenient to halve these quantities. 



rise gradually in pH from the time when they are made up. Thus 

 Helly rises from pH 3-70 when fresh to pH 4-20 on the next day. 

 The oxidation-potential falls at the same time; that of Sanfelice 

 falls even more. Other fixative mixtures, such as Altmann, Bouin, 

 Champy, Susa, Hermann, Perenyi, and Zenker, maintain a con- 

 stant or nearly constant pH, and at least some of them (Altmann, 

 Champy, Zenker) a constant oxidation-potential; Flemming with 

 full acetic becomes more acid on standing. 



The products of the reactions in unstable mixtures appear not 

 to have been worked out. The reduction of the chrome anions is 

 likely to produce cationic chromium; this would itself act as a 

 fixative, though quite differently from the anions. The use of 

 cationic chromium in fixation has been suggested by Zirkle.^^^ If 

 unstable mixtures are really necessary, it is important that their 

 reaction-products and the effects of these on the tissues should be 

 known. 



Of the fixatives listed in table 8, nearly all that are used chiefly 

 in micro-anatomy, embryology, histolog}^ and in studies of 

 chromosomes contain acetic acid. This applies also to Clarke and 

 Carnoy. The only exceptions are Kleinenberg (which contains 

 sulphuric acid) and Helly. For chromosome studies acetic acid is 



