ISOTONIC SALINE AND DISTILLED WATER AS SOLVENTS 507 



up ill normal saline than in distilled water. When dissolved 

 in the latter the ground cytoplasm is often vacuolated, and, 

 sometimes, partly destroyed. 



Subject to revision in the light of further observations, 

 I suggest the following as an explanation of the distortion of 

 tissues caused by fixation in 5 per cent, formol made up in 

 distilled water. As already pointed out, the only effect of 

 dissolving mercuric chloride in normal saline is sliglitly 

 to increase the molecular concentration of the mixture. But 

 in the case of formol this is different, for the low molecular 

 concentration of 5 per cent, solution of formol (as compared 

 to 6 per cent. HgClg) is ajypreciahly increased by making 

 it up in normal saline instead of distilled water. This means 

 that while a given concentration of formol in normal saline 

 may be isotonic with tissues, the same concentration of 

 formol in distilled water may be sufficiently hypotonic to 

 cause distortion and swelling of cells. 



Solutions of mercLiric chloride and formol when dissolved 

 in hypertonic solutions of saline of double the normal strength 

 give rise to tissue-shrinkage. This shrinkage is more marked 

 in the formol than in the mercuric chloride series. Further, 

 the degree of shrinkage induced by the fixative varies greatly — 

 as is well known — with different tissues. 



These remarks concerning the dilution of mercuric chloride 

 and formol in distilled water and in isotonic saline only apply 

 to these two fixing reagents used in the concentrations already 

 mentioned, these being, moreover, the concentrations at which 

 they are the most commonly employed. The question of the 

 tonicity of compound fixatives is omitted, for here, as claimed 

 by Gatenby (1 and 2), the strength of the fixative is regulated 

 by diluting it, if necessary, with distilled water. Thus, if 

 fixation in a chrome-osmium mixture produces cell-shrinkage, 

 the fixative should be diluted with a known volume of distilled 

 water. This trial and error method is repeated until the dilution 

 of the fixative is such that it does not cause shrinkage by 

 a too rapid exosmosis from the cells. 



