150 FIXATION 



way that it readily admits paraffin, makes it strongly acidophil, 

 and fixes chromosomes rather well. It has, however, two serious 

 defects: it shrinks tissues badly and makes chromatin acidophil. 

 Acetic acid compensates for both these defects. 



Unfortunately the good qualities of primary fixatives cannot 

 always be combined in mixtures. Potassium dichromate tends to 

 stabilize the cytoplasm and nuclear sap in a homogeneous state, 

 but dissolves and disperses chromatin: acetic acid fixes chromatin 

 but does not fix cytoplasm or nuclear sap. The attempt was 

 naturally made to let each compensate for the defects of the other. 

 Yet Tellyesniczky's fluid *^^ did not and could not achieve its 

 objects, because the acidification of the chrome anions causes 

 them to act as though chromium trioxide had been dissolved 

 instead of potassium dichromate, and this is a strongly coagulative 

 fixative that does not stabilize cytoplasm or nuclear sap as a 

 homogeneous gel. 



In deciding which primary fixatives to mix, it is important to 

 take into account not only their obvious mutual compatibilities and 

 incompatibilities, but also their more subtle influences on one 

 another's properties. Thus anyone who includes mercuric chloride 

 in a mixture should remember that its coagulative power and the 

 solubility of its coagulates are affected by acidification (p. 52). 



The effect of so-called indifferent salts must also be con- 

 sidered. Sodium chloride, as we have seen (p. 54), can dissolve 

 protein coagulates produced by mercuric chloride, in certain 

 circumstances. Ammonium sulphate can transform ferric sulphate, 

 at certain concentrations, from a non-coagulant to a coagulant 

 fixative (p. 85). Indifferent salts can be very useful ingredients of 

 mixtures, but it is unlikely that they improve any fixative that 

 contains acetic acid at 5% or thereabouts. 



All the fixative ingredients of an ideal mixture would penetrate 

 at the same speed. This could be achieved by adjustment of their 

 concentrations. It does not appear that any inventor of a fixative 

 mixture has taken this into consideration. A section through a 

 large piece of tissue often gives unmistakeable evidence that one 

 ingredient reached the centre at fixative concentration in advance 

 of the others. When a small piece of tissue is used, the unevenness 

 due to this cause is minimized. The argument presented in detail 

 on p. 69 applies here. When a piece of tissue has been placed in the 

 fixative mixture, each ingredient must be supposed instantly to 

 reach the opposite side of it from that at which it started, at in- 



