422 COLLOIDS IN BIOLOGY AND MEDICINE 



Acetic acid alone (in concentration up to 1 per cent) causes swelling, 

 but is suitable for combination with reagents which cause shrinking, 

 especially for the fixation of nuclear structures. 



Trichloracetic acid (5 per cent to 10 per cent) penetrates 

 rapidly, destroys the most delicate structural relations of pro- 

 toplasm and nucleus but fixes well centrosomes, chromosomes and 

 spindles. Since fibrillar connective tissue swells strongly in tri- 

 chloracetic acid, the preparation must be placed at once in absolute 

 alcohol. 



Picric acid does not change all sols and reversible gels into irre- 

 versible gels. This follows from the results of A. FISCHER who 

 showed that precipitations with picric acid are dissolved again by 

 water; and this is confirmed by the experience of histologists with 

 actual specimens. Only when combined with other acids (acetic 

 acid, chromic acid, sulphuric acid, nitric acid and osmic acid) does 

 picric acid attain its importance as a fixative,, and under these cir- 

 cumstances it is highly praised. 



Since picric acid is lipoid soluble and forms insoluble dye salts 

 with dye bases it is suitable for fixing specimens after "vital stain- 

 ing." 



Salts. Among these, the chlorids next to the chromates enjoy 

 especial popularity. I attribute this to their ease of diffusion and to 

 the fact that in respect to swelling and shrinking, the chlorin ion 

 occupies approximately a middle position. 



Copper chlorid and copper acetate are suitable for delicate lower 

 plants but are very seldom employed. 



Mercuric chlorid in concentrated aqueous solution is very suitable 

 for the fixation of animal preparations. I have had very good re- 

 sults in fixing leucocytes. It may not be employed for any molluscs 

 or for fresh water crustaceans; it also seems not quite suitable for 

 plant cells. As a result of a certain amount of lipoid solubility it is 

 able to penetrate the living cells and on this account in vital stain- 

 ing it serves to fix the dye. Obviously, part of the mercuric chlorid 

 is in this case bound by the protoplasmic albumin; the resulting 

 combination is somewhat insoluble in water. 



Ferric chlorid in alcoholic solution is recommended for pelagic 

 marine animals. 



Platinum chlorid (0.1 to 1 per cent), palladious chlorid (0.1 per 

 cent) and iridium chlorid are recommended for special purposes (usu- 

 ally in mixtures). 



Potassium bichromate is rarely used as a pure solution, since it 

 markedly changes the structure, but used in combination with other 

 substances it is a very popular fixative (with acetic acid for cell sub- 



