REACTIONS OF FIXATIVES WITH TISSUES AND CELLS 77 



shrunk by after-treatment. Picric acid shrinks strongly and permits 

 much subsequent shrinkage. Mercuric chloride and formaldehyde 

 gave less final reduction in volume than any other primary (un- 

 mixed) fixative that wsls tested. 



Hooks may be inserted in each end of a piece of liver or other 

 tissue, and a string from one of them may be attached to a lever; 

 this will record on a revolving drum the progressive changes in 

 linear dimensions that occur at every stage from fixation up to 

 embedding in melted paraffin. ^^^ Experiments carried out in this 

 way show that in routine microtechnique there are three stages at 

 which shrinkage chiefly occurs after fixation. There is sudden 

 shrinkage of mammalian organs on passing from 70% to 90% 

 ethanol (reminiscent of the shrinkage of Pleiirobrachia on passing 

 from 60% to 70%), a gradual but pronounced shrinkage in xylene, 

 toluene, or other antemedium, and a further prolonged shrinkage 

 in hot paraffin. (Liquid paraflin at room temperature also shrinks.) 

 These, then, are the particular stages in embedding against which 

 fixatives do not give protection. There are obvious advantages in 

 methods of embedding that evade some or all of the stages at 

 which shrinkage chiefly occurs. It is to be noted that absolute 

 ethanol does not have much effect on the volume of fixed tissues 

 that have already been soaked in 90%. 



It is particularly important that we should know how fixation 

 and the subsequent processes of microtechnique affect the volumes 

 of individual cells. Those of regular form naturally commend 

 themselves for work of this kind, because they are easy to measure. 

 The eggs of cows,^^^ rabbits, ^^^ and echinoids ^^^ have been chosen 

 for this reason ; also the fully-grown primary spermatocytes of the 

 snail {Helix asper'sa).^'^^ The possible sources of error in this kind of 

 investigation have been best appreciated by Ross,^^^ who, in his 

 recent work at Oxford, has made every effort to avoid them and 

 has even taken into consideration the change of magnification of 

 objectives when mounting media of different refractive indices are 

 used. He has expressed his results in the form of medians and 

 shown the scatter of his observations by the use of frequency 

 polygons, while the others have relied on the means of relatively 

 few measurements. 



The diameters of the cells can be measured while they are alive 

 in sea-water or other appropriate ffuid, and at any number of 

 subsequent stages in the routine processes of microtechnique. 

 Hertwig's ^^^ investigation was the most complete in this respect. 



