REACTIONS OF FIXATIVES WITH TISSUES AND CELLS 83 



TABLE 7 



Grading of simple fixatives with and without the addition of indifferent 

 salts. ^^ The fixatives were used at the concentrations shown on p. 24. 



indifferent salt, and most of the others sHghtly ; with trichloracetic 

 and acetic acids, however, there was no difference. 



The facts, then, are clear enough. Indifferent salts do improve 

 fixation by certain fixatives, especially the fixation of the cells of 

 marine algae and marine invertebrates. The reason for this is not 

 obvious. More than half a century ago the Swedish investigator 

 Sjobring *^* laid it down that formaldehyde should be used at such 

 a concentration as to make the fluid isotonic with the tissues to be 

 fixed. For mammalian tissues he used formaldehyde at 8 to 10%. 

 He should actually have used it at about 4% to achieve his pur- 

 pose,^^^ but the question is whether his principle was correct. The 

 evidence against it is conclusive. ^^^' ^^^' ^^ Fixatives may be made up 

 so as to be hypotonic, isotonic, or hypertonic to the body-fluids: 

 the results show that there is no virtue in isotonicity.^^^ Acetic 

 acid at 5% swells tissues, yet it exerts an osmotic pressure of 20 

 atmospheres, about three times that exerted by mammalian blood. 

 Chromium trioxide at ^%, with the very small osmotic pressure 

 of 1-4 atmosphere, shrinks cells strongly. Similar examples could 

 be multiplied. 2^ It is clear that the total osmotic pressure exerted 



