56 CYTOLOGICAL TECHNIQUE 



FORMALDEHYDE IN SIMPLE FIXATIVES 



When used in this way, formaldehyde is generally dissolved in 

 a solution of an indifferent salt. The latter is used at a concentra- 

 tion that gives the same osmotic pressure as the body-fluids of the 

 organism from which a part is to be taken for fixation, or at a 

 concentration slightly less than this. The following is suitable for 

 the tissues of most vertebrates other than elasmobranchs, and for 

 many terrestrial and fresh- water invertebrates : 



Distilled water 83 ml 



Sodium chloride, 10% ^4- • • . 7 ml 

 Formalin . . . . . . 10 ml 



Keep marble in the fluid. pH (in the presence of marble), 6-0. Wash 

 out with water or 50% ethanol. 



In this and other formulae, the word 'formalin' means the com- 

 mercial fluid containing formaldehyde at approximately 40% 

 w/v. In the fixing solutions given here, formaldehyde is therefore 

 used at its standard concentration. 



For the tissues of marine invertebrates, one may dilute formalin 

 with 9 times its volume of sea-water. The pH of the fluid is 7-6. 



There can be no doubt that non-fixative salts improve fixation 

 by formaldehyde and certain other primary fixatives. Since they 

 are used at concentrations that give about the same osmotic 

 pressure as the body-fluids of the organism, the fixative solution 

 as a whole is hypertonic, for the osmotic pressure of the formalde- 

 hyde or other fixative is added to that of the non-fixative salt. 

 One would therefore expect the latter to cause shrinkage: in fact, 

 however, the virtue of non-fixative salts is that they reduce shrink- 

 age. It has already been mentioned (p. 29) that this has never been 

 satisfactorily explained. It has been suggested that when fixatives 

 are used without indiff"erent salts, the cells in the interior of a 

 piece of tissue behave at first— before the fixative substance itself 

 has arrived— as though the piece had been placed in distilled 

 water: that is to say, they swell up and burst. After bursting, they 

 are thought to shrink, and to be fixed subsequently in this 

 shrunken condition. If an indifl'erent salt is used, it diffuses into 



