REACTIONS OF FIXATIVES WITH TISSUES AND CELLS 75 



rely on fixatives in the higher grades, when the embedding and 

 mounting media to be used are the same as in the test. 



It is wrong to speak of good and bad fixatives without mention- 

 ing the after-treatment. A fixative that stabiUzes structure against 

 embedding in paraffin is hkely to give good results with milder 

 after-treatment, but one that works well with mild after-treatment 

 may give poor results with paraffin. Flemming's weak fluid ^^^ 

 seems a very poor fixative (grade IV-V) when paraffin sections are 

 used. It should be remembered that the great cytologist left 

 minute pieces of tissue or separate cells in his fluid for half an 

 hour and then examined them in water without any other treat- 

 ment whatever. There was no question of embedding, and he 

 admitted that his preparations lost some of their delicacy if 

 mounted in glycerine. In fact, he used his weak fluid rather as a 

 preservative than as a fixative. Ordinary formaldehyde/saline 

 (formaldehyde at 4% in 0-7% aqueous sodium chloride solution) 

 gives poor results (grade III-IV) with paraffin sections, but quite 

 good (grade II) with collodion. An extreme example of the same 

 kind is provided by Palade's buffered osmium tetroxide solution. ^^^ 

 This fixative is one of the best available to the electron-micro- 

 scopist when tissues are embedded in methacrylate, but the results 

 with paraffin embedding are very poor (fig. 9, b). 



Non-quantitative tests, including the one we have been dis- 

 cussing, have considerable value. Thus it is important to know 

 that one fixative destroys mitochondria, a second leaves them 

 undamaged but unfixed, and a third stabilizes them against paraffin 

 embedding. A multitude of similar examples could be quoted. 

 Still, it is to be wished that we had more knowledge that could be 

 expressed in numbers. Our main fund of quantitative information 

 concerns shrinkage and swelling. 



The length and breadth of a whole organism may be measured 

 before and after fixation, and at various subsequent stages of 

 treatment. This can be useful in various ways (for instance, by 

 enabling us to judge the initial size and therefore the age of em- 

 bryos that have been fixed) ; ^^^ but it provides little knowledge 

 about the action of fixatives, since shrinkage or swelling may 

 affect mainly the cells or mainly the body-cavity and other inter- 

 cellular spaces, or a vertebral column may interfere with changes 

 in length that would have occurred in its absence. Soft organisms 

 with relatively small internal cavities do, how-ever, provide useful 



