REACTIONS OF FIXATIVES WITH TISSUES AND CELLS 85 



results. Perhaps the salt acts on the proteins of the cell in such a 

 way as to affect the imbibition of water. Some protein gels have a 

 tendency to swell in the presence of non-fixative salts, in much the 

 same way (though not so much) as they swell in the presence of 

 acetic acid; this would counteract the shrinking effect of certain 

 fixatives. It is relevant to remark that when acetic acid is included 

 in a fixative, the presence or absence of an indifferent salt has 

 little effect. ^^^ The fact must be kept in mind, however, that 

 echinoid eggs swell in the absence of an indifferent salt. 



If an indifferent salt acts within the cell, it does not necessarily 

 do so by affecting imbibition. Its influence may be on the reaction 

 of the fixative substance with the proteins of the cell. We have 

 already seen how the action of mercuric chloride on proteins is 

 affected by the presence of sodium chloride (p. 54). A still more 

 striking instance is provided by ferric sulphate, w^hich is rather a 

 useful fixative. ^^ By itself, a J% solution of the anhydrous salt 

 does not coagulate egg- albumin; in the presence of ammonium 

 sulphate it coagulates it instantly (see Appendix, p. 316). Ammo- 

 nium sulphate alone is devoid of the ability to coagulate egg- 

 albumin at any concentration, and it is therefore an indifferent 

 salt. 



If indifferent salts exert their effects after entering cells with the 

 fixative substance, it does not follow that their osmotic pressure is a 

 matter of indifference. Such a salt may diffuse rapidly into the piece 

 of tissue and act osmotically, in the way suggested, until the fixative 

 substance, diffusing more slowly, reaches the cells in the interior; 

 it may then enter the cell with the fixative when the latter has 

 damaged the cell-surface and stopped it from acting as a semi- 

 permeable membrane. 



It follows from what has been said that the use of indifferent salts 

 is reasonable, though acetic acid may in some cases replace them ; 

 that they should not be used indiscriminately, without considera- 

 tion of their possible effect on the coagulative powers of fixative 

 substances; and that they should be present at such a concentra- 

 tion that they exert about the same osmotic pressure as the inter- 

 cellular fluids of the tissues. 



It has not been proved that the osmotic pressure of the indifferent 

 salts should be exactly the same as that of the body-fluids. A 

 slightly lower concentration seems to give better results. Thus 

 sodium chloride works well at 0-7% with the tissues of mammals, 

 and calcium chloride at 1%.^^ 



