CHAPTER 111. 25 



For routine zoological work Bourn's picro-formalin-acetic is 

 recommended. Gilson-Petrunkewitsch is a fixative which is easy 

 to work and generally better than corrosive sublimate acetic. 



For routine vertebrate histological work Zenker and Helly's 

 Zenker-formol are indicated. 



I think the beginner should avoid such things as liquid of FLEMMING 

 and similar mixtures. 



Picric acid gives a fair though weak fixation, with very good 

 penetration, is easy to manage, and does not make tissues brittle, 

 which sublimate easily may do. Pure formol is not bad, and very 

 easy to manage. 



Speaking generally, osmic acid, chromic acid, bichromates, chloride 

 of platinum, and the majority of the compounds of the heavy metals, 

 are hindrances to staining ; whilst heat, alcohol, trichloracetic acid, 

 formol, corrosive sublimate, picric acid, and acetic acid, are neutral, or 

 even favourable, in this respect. 



31. The Practice of Fixation. See that the structures are per- 

 fectly living at the instant of fixation, otherwise you will only fix 

 pathological states or post-mortem states. 



Some observers have made special observations on the effect of delay in 

 fixation; J. THORNTON CARTER (Phil. Trans. Roy.Soc., Series B,vol. ccviii, 

 1917) has made some interesting experiments on the finely granular 

 ameloblasts in the developing teeth of the pike. He noticed that the 

 cytoplasm gave evidence of marked changes unless fixed within three 

 minutes of " death " ; these changes were manifested by the behaviour 

 of the cytoplasmic granules to stains ; the selectivity of the latter was 

 progressively altered as the rapid post-mortem changes were set in 

 action. 



Fixation is generally performed by immersion of the objects in 

 the fixing liquid. In this case, everything should be done to facilitate 

 the rapid penetration of the fixing agent. To this end let the struc- 

 tures be divided into the smallest portions that can conveniently be 

 employed, and if entire organs or organisms are to be fixed whole, 

 let openings, as large as possible, be first made in them. 



The penetration of reagents is greatly facilitated by heat. You 

 may warm the reagent and put it with the object to be fixed in the 

 paraffin stove, or you may even employ a fixing agent heated to 

 boiling-point (as boiling sublimate solution for certain corals and 

 Hydroids, or boiling absolute alcohol for certain Arthropods with 

 very resistent integuments). But this should only be done as a last 

 resource. 



Let the quantity of fixing agent employed be many times 



