58 FIXATION 



chromium trioxide with the tissues, and may then perhaps itself 

 react with the tissues. 



We turn now to the non-coagulant additive fixatives, formalde- 

 hyde and osmium tetroxide. These are of particular importance, 

 for the very fact that they are non- coagulant makes them unlikely 

 to distort tissues and cells seriously. It must nevertheless be kept 

 in mind that subsequent treatment, especially embedding in 

 paraffin, often results in gross distortion. 



Formaldehyde can be caused to form compounds with various 

 amino-acids,^"^ but most of these reactions appear to be irrelevant 

 to microtechnique. The compound with tyrosine, for instance, is 

 only formed by heating for several hours in acid solution. *^^ 



The reactions of formaldehyde with polypeptides, each con- 

 sisting of chains of only one particular amino-acid, are extremely 



NH NH NH 



HCH HC(CH2)2C\ HC(CH2)2C( 



I I ^OH I \NH2 



Glycine Glutamic acid Glutamine 



The amifio-acids of poly glycine, polyglutamic acid, and polyglutamine 



instructive. Experiment shows how much formaldehyde such 

 polypeptides will remove from solution. ^"^ Polyglycine binds very 

 little formaldehyde; so does polyglutamic acid. Polyglutamine, on 

 the contrary, binds more formaldehyde than any other macro- 

 molecule, so far as is known. 



Silk-fibroin consists mainly of alanine and tyrosine. Like poly- 



NH 

 HC.CH3 alanine 



:—o 



NH 



HC.CHaC^ ^ OH tyrosine 



f 



=0 



? 



Part of a molecule of silk-fibroin 



glycine and polyglutamic acid, it binds very little formaldehyde: 

 less than one-twentieth as much as polyglutamine. 



