84 FIXATION 



by the constituents of a fixative is unrelated to the sweUing or 

 shrinkage of cells. 



According to several authors,^"' *^^' ^^*' ^^^ fixative substances 

 should be dissolved in a solution of an indifferent salt giving an 

 osmotic pressure equal to that of the intercellular fluids of the 

 tissues that are to be fixed. It follows that the fixative solution as a 

 whole should be hypertonic to these fluids. 



It is claimed that when a piece of tissue is placed in a solution of 

 a fixative substance dissolved in distilled water, the cells in the 

 centre of the piece are affected as they w^ould be if it had been 

 placed in distilled water. The ions of the intercellular fluid diffuse 

 rapidly outwards into the fixative solution, while the fixative sub- 

 stance penetrates more slowly inwards. The intercellular fluid 

 therefore becomes hypotonic to the cells, and the latter accord- 

 ingly swell and burst. If the fixative is dissolved in a suitable saline 

 medium, the mobile ions of the latter counteract this process by 

 diffusing into the tissues ahead of the fixative substance. ^^^ It is 

 to be noticed that this theory postulates the presence of an inter- 

 cellular fluid. Now we have already seen that echinoid eggs swell 

 strongly when placed in a formaldehyde solution devoid of in- 

 different salts, but scarcely at all when this fixative is dissolved in 

 sea- water. Thus the initial swelling, postulated as the prime cause 

 of the damage, occurs also when there is no intercellular fluid. 

 Beyond this, it is to be held in mind that no one has proved that 

 cells do in fact burst when placed in fixative solutions lacking 

 indifferent salts. They appear shrunken in the final preparation. It 

 is not clear why a burst cell must shrink. 



It is argued that fixative substances can pass easily through cell- 

 membranes, and that they are therefore not able to exert osmotic 

 pressure on the cell-contents. ^^^ It has also been argued that the 

 cell-surface is so much altered by fixation that it is no longer able 

 to act as an osmotic membrane when a fixative has reached it.^^^ 

 Hertwig's experiments with echinoid eggs do not appear to support 

 these opinions. Formaldehyde comes up against the cell- membrane 

 directly the eggs are put in the solution, yet there is swelling unless 

 the fixative was dissolved in sea- water. This suggests that the cell- 

 surface continues to act as an osmotic membrane. 



If the indifferent salt acts osmotically, it must perform its 

 function while remaining outside the cells. We have no proof that 

 in fact it remains outside. On the contrary, it is conceivable that 

 it enters the cells with the fixative and then produces the observed 



