INTRODUCTION TO FIXATION 29 



of the sections. Care should be taken to chose deHcate tissues as 

 test- objects, for some are so robust that they withstand the action 

 of indifferent fixatives. It would be proper to take several bits of 

 material fixed in each of the fluids that it is desired to compare, 

 and embed them in a variety of different media ; for a fixative that 

 gives good results with one embedding medium may fail badly with 

 another. A full investigation of this sort has never been undertaken, 

 but details of a test involving paraffin embedding will be given on 

 a later page (p. 72). Paraffin is probably the most drastic medium. 



The minimum length of time during which a fixative should act 

 depends partly on its rate of penetration (see pp. 37, 67) and partly 

 on its mode of action. In general, the coagulant fixatives have 

 achieved their full effect at any particular depth in the tissue as soon 

 as they have penetrated to that depth at the concentration necessary 

 to cause coagulation; but some fixatives, especially formaldehyde 

 (p. 60), may continue to act progressively on the tissues after the 

 latter has been fully permeated. Unless a fixative produces an ex- 

 trinsic artifact, prolonged action is seldom harmful. Tissues may 

 be left at least three years in Bouin's fluid without harm.^^ It is 

 convenient as a general rule to fix overnight (about 18 hours). 

 Very much shorter periods are sufficient for the tiny pieces, only 

 about a millimetre thick, that are often used in cytological work; 

 but overnight fixation ordinarily causes no damage. If the proteins 

 of the interior of a piece of tissue have not been fully acted upon in 

 18 hours, it is unlikely that satisfactory fixation will be achieved 

 by prolonging the period. For this reason it is not sensible to try to 

 fix pieces of tissue several centimetres thick. 



Careful instructions are often given as to the exact length of 

 time during which particular fixatives should act, but these may 

 often be disregarded. The truth of this may be established by 

 getting someone else to fix three similar bits of tissue in the same 

 fixative for 8, 16, and 32 hours. When these have been sectioned, 

 dyed, and mounted, it will be found difficult or impossible to 

 guess which is which, unless there happens to have been pro- 

 gressive deposition of an extrinsic artifact and this has not been 

 removed. 



It is sometimes desirable to 'postchrome' or 'postosmicate' a 

 piece of tissue ; that is to say, to leave it for a long time in a solution 

 of potassium dichromate or osmium tetroxide after initial fixation. 

 (See pp. 129 and 126.) 



