76 FIXATION 



information, for shrinkage is usually uneven and therefore pro- 

 duces obvious changes of form. The ctenophore Pleurobrachia is a 

 delicate organism, well suited to observations of this kind. If 

 fixed in a solution of formaldehyde in sea-water, it retains its 

 shape well. If it now be transferred to 10% ethanol and thence 

 slowly through 20%, 30%, 40%, and so on up to absolute ethanol, 

 the main shrinkage occurs in 70%, with obvious deformation.^^ The 

 lower ethanols, up to 60% or thereabouts, appear not to cause 

 much shrinkage of properly fixed tissues, but there is always a stage 

 in dehydration by ethanol, somewhere in the higher concentrations, 

 against which no known fixative will protect the tissues. 



Accurate measurements may be made of changes in volume 

 undergone by whole organs, ^^^' ^^ or by large cubes cut from whole 

 organs. ^*^ The liver and spleen are especially suitable because they 

 are fairly homogeneous in structure and contain no large empty 

 spaces. The volume may be recorded when the organ is fresh and 

 again at any number of subsequent stages up to and including 

 infiltration with paraffin. The results of experiments of this sort 

 with liver are shown in table 4. It will be noticed that the change in 



TABLE 4 



The effect of fixation and subsequent treatment on the volume of whole 



livers {? mammalian). 

 [Means of several observations in each case; rearranged from the data 



of Berg. ^'^) 



volume cause by fixation itself gives little indication of the total 

 shrinkage that will have occurred when the organ has been in- 

 filtrated with paraffin wax. Potassium dichromate, for instance, 

 causes no change of volume but allows the tissue to be excessively 



