58 CYTOLOGICAL TECHNIQUE 



on the alkaline side of neutrality, though this has been disputed. 

 The buffer may also act as an indifferent salt. A modification of 

 Michaelis's buffer^^a jg usually used, probably because it is not 

 harmful to the cells of most vertebrates. The buffer at pH 7-4 is 

 this: 



Sodium acetate and sodium veronal, both 



M 



at — m one solution, aq. . . . 20 ml 

 7 ^ 



Hydrochloric acid, 0-1 N. . . .20 ml 



Sodium chloride, 8-5 aq. . . . . 8 ml 



Distilled water . . . . . 52 ml 



The acetate- veronal solution for use in this buffer and in Palade's 

 fixative (see below) is made by dissolving 9-714 g of sodium acetate, 

 SHoO, and 14-714 g of sodium veronal in distilled water, and making 

 up the volume to 500 ml by the addition of distilled water. 



Michaelis's buffer has approximately the same osmotic pressure 

 as the body-fluids of mammals. Sodium veronal is a useful con- 

 stituent of buffers intended for biological work in the neighbour- 

 hood of neutrality. Boric-borate buffers are troublesome, because 

 boric acid forms complexes with carbohydrates and all other 

 organic di- and polyhydroxy-compounds. Sodium acetate acts in 

 buffers at a much lower pH than 7-4, and an equally good solu- 

 tion lacking this constituent could probably be devised. 



Palade's fixative^ ^^ is Michaelis's buffer at pH 7-4, with the 

 addition of 1 g of osmium tetroxide and the substitution of dis- 

 tilled water for the sodium chloride solution. Thus it contains 

 osmium tetroxide at its standard concentration, and the saline 

 constituents are hypotonic to the body-fluids of mammals. It is 

 convenient to make up Palade's fixative thus: 



Sodium acetate and sodium veronal, both 



M 



at — - in one solution (see above) . . 20 ml 



7 



Hydrochloric acid, 0-1 N . . .20 ml 



Distilled water . . . . . 10 ml 



To 1 ml of the fluid, add 1 ml of 2^'o osmium tetroxide solution. 

 pH of the fixative solution, 7-4. 



