18 FIXING AGENTS. 



27. Fixation by the Vapours. Osmic acid is frequently em- 

 ployed in the form of vapour, and its employment in this form 

 is indicated in most of the cases in which it is possible to 

 expose the tissues directly to the action of the vapour. The 

 tissues are pinned out on a cork which must fit well into a 

 wide-mouthed bottle in which is contained a little solid osmic 

 acid (or a small quantity of 1 per cent, solution will do). 

 Very small objects, such as isolated cells, are simply placed on 

 a slide, which is inverted over the mouth of the bottle. They 

 remain there until they begin to turn brown (isolated cells will 

 generally be found to be sufficiently fixed in thirty seconds, 

 whilst in order to fix the deeper layers of relatively thick 

 objects, such as retina, an exposure of several hours may be 

 desirable). It is well to wash the objects with water before 

 staining, but a very slight washing will suffice. For staining, 

 methyl-green may be recommended for objects destined for 

 study in an aqueous medium, and, for permanent preparations, 

 alum-carmine, picro-carmine, or haematoxylin. 



The reasons for preferring the process of fixation by vapour 

 of osmium, where practicable, are that osmium is more highly 

 penetrating when employed in this shape than when employed 

 in solution, and produces a more equal fixation, and that the 

 arduous washing out required by the solutions is here done 

 away with. In many cases delicate structures are better pre- 

 served, all possibility of deformation through osmosis being 

 here eliminated. 



In researches on nuclei, it is possible and may be useful to 

 employ the vapours of a freshly-prepared mixture of osmic 

 and formic or acetic acid (G-ilson, La Cellule, i, 1885, p. 96). 



28. Fixation by Solutions. When employed in aqueous solu- 

 tions osmic acid is used in strengths varying from -$ per cent, 

 to 2 per cent. Solutions of \ per cent, to 1 per cent, have 

 been very largely used, but the tendency of modern practice 

 seems to be towards weaker solutions and longer immersion. 

 For Infusoria J per cent, for a few seconds ; for Porif era -^ 

 to yL. per cent, for some hours ; for Mollusca 1 to 2 per cent, for 

 twenty -four hours ; for epithelia -^ to per cent, for an hour 

 or two; for meroblastic ova -$ per cent, for twenty-four 

 hours ; for medullated nerve-fibre ^ to 1 per cent, for from 

 twenty minutes to two hours ; for tactile corpuscles | to 1 per 



