REACTIONS OF FIXATIVES WITH TISSUES AND CELLS 79 



The evidence suggests that nuclei generally shrink less than 

 whole cells. Some of Hertwig's results with nuclei are shown in 

 fig. II. It will be noticed that infiltration with melted paraffin 

 causes further shrinkage of nuclei bevond that caused bv dehvdra- 

 tion and soaking in xylene. 



Shrinkage is harmful partly because it always involves distortion, 

 partly because structural detail that approaches the limit of resolu- 



LIVING CELL 



FORMALDEHYDE. MERCURIC CHROMIUM ACETIC POTASSIUM PICRIC ETHANOL, 



4% CHLORIDE, TRIOXIDE. ACID, DICHROMATE, ACID, ABS. 



SAT. 0-75% 5% 5% SAT. 



FIG. 10. Outlines of the fully-grown primary spermatocyte of the 

 snail, Helix aspersa, to show the effect of fixation and subsequent 

 treatment on the size of the cell. All the cells except the one at the 

 top have been fixed, embedded in paraffin, sectioned, dyed, and 



mounted in Canada balsam. 



(Diagrammatic but accurately to scale: from the data of Ross.***) 



tion of the microscope may pass beyond it. It is not surprising 

 that paraffin wax is so much favoured by microtechnicians, for 

 serial sections can be obtained very easily and their attachment 

 to slides is quick and simple ; but there is probably no other method 

 of embedding that involves so much shrinkage. Our familiar fixa- 

 tives may perhaps owe their survival to the fact that they give the 

 best (or least bad) results with paraffin embedding. (See p. 148.) 

 There is room for much experiment here. We need new fixa- 

 tives that will stabilize tissues better against existing after-treat- 

 ments, and new after-treatments that will cause less shrinkage, or 

 preferably none at all. The stearates of diethylene glycol present 

 considerable advantages in this respect. *^^' ^^^ The polyethylene 

 glycols ('carbowax') ^^^' ^^^'^1 are particularly promising em- 

 bedding media, because tissues can be passed directly into them 

 from water and shrinkage seems to be slight; but there are still 



