64 CYTOLOGICAL TECHNIQUE 



The fluid differs from the mixtures already described and from 

 the great majority of fixative mixtures used in microtechnique, in 

 containing no coagulant. Ground cytoplasm is therefore fixed 

 very homogeneously. The cytoplasmic inclusions maintain their 

 form, partly because they are not distorted by coagulation of the 

 surrounding ground cytoplasm, partly because both the constitu- 

 ents of the fluid are lipid-fixatives. The dichromate, a stronger 

 oxidizer than osmium tetroxide, prevents excessive blackening of 

 the tissues and gives easy colouring by acid dyes ; unfortunately it 

 dissolves nucleoprotein, and the fixative is not adapted to studies 

 of nucleus or chromosomes. 



The disadvantage of this fixative is that it does not give ready 

 access to melted paraffin. Embedding often results in shrinkage 

 and distortion, the tissue cracks, and sections are difficult to 

 flatten. When skill or luck overcomes these troubles, excellent 

 preparations result. 



Ammonium dichromate may be substituted for potassium di- 

 chromate in this solution."^ The pH of 'NH4-Altmann' is slightly 

 more acid. The ammonium salt differs curiously from the other. ^^^ 

 Mitochondria are not well fixed at the surface of the piece of 

 tissue, but in the interior they are better fixed than by potassium 

 dichromate: filamentous ones retain their form, instead of 

 shortening and thickening. For this reason it is best to cut out 

 rather large pieces of tissue when the ammonium salt is to be 

 used. 



The great majority of fixative mixtures fall into one or other of 

 four groups. Fifteen of the most valuable are listed here. 



Group A. Coagulant + acetic acid. These are primarily fixa- 

 tives for micro-anatomy and histology. Examples: Clarke,^^ 

 Zenker. ^^^ 



Group B. Coagulant -f- non-coagulant + acetic acid. These are 

 mostly fixatives for detailed histology and general cytology : many 

 are used in the study of chromosomes. Examples: Flemming's 

 strong fluid,'^ Allen's 'B.15',i Bouin,=^9 Heidenhain's 'Susa',^« 

 Hermann (mammalian formula),^^ Sanfelice.^^^ 



Group C. Coagulant -f non-coagulant. These are routine fixa- 



