78 FIXATION 



Paraffin embedding has always been used. It is desirable that 

 comparative studies should be made with other media. 



Fixation does not necessarily shrink cells, and may indeed swell 

 them strongly, but dehydration always reduces the volume below 

 that of the living cell, and immersion in xylene carries the process 

 still further. Some typical examples are given in tables 5 and 6. 



TABLE 5 



The volwries of unfertilized eggs 0/ Arbacia pustulosa at various stages of 

 embedding in paraffin, expressed as percentages of the volumes zvhile alive 

 in sea-water. Each figure is calculated from the mean diameter of 10 to 15 



eggs. [Data of Hertzvig.^^^) 



TABLE 6 



The volumes of the pritnary spermatocytes of the snail, Helix aspersa, fixed 



in various simple fixatives, embedded in paraffin, dyed, and mounted in 



Canada balsam. They are expressed as percentages of the volume of the 



living cell. {Rearranged from the data of Ross. ^'^^) 



Shrinkage always occurs when cells fixed in aqueous media are 

 transferred to 70% ethanol, and further shrinkage at each sub- 

 sequent stage, at an}^ rate up to xylene. We unfortunately have no 

 data on the change of volume of whole cells on passing from 

 xylene to melted paraffin. 



A comparison of table 4 with tables 5 and 6 suggests that in- 

 dividual cells are more shrunken than whole organs by the pro- 

 cesses of routine microtechnique. It is doubtful whether most 

 histologists and cytologists realize the extreme degree of com- 

 pression that a cell has undergone when it is examined in an 

 ordinary microscopical preparation. (See fig. 10.) 



