42 FIXING AND HARDENING AGENTS 



latter the ground cytoplasm is often vacuolated, and sometimes 

 partly destroyed. 



For fixing, corrosive sublimate may be used pure ; but in most 

 cases a finer fixation will be obtained if it be acidified with acetic 

 acid, say about 1 per cent, of the glacial acid. We find that a 

 saturated solution in 5 per cent, glacial acetic acid is a very good 

 formula for marine animals ; for others take the acid weaker. 

 Kaiser's solution consists of 10 grm. sublimate, 3 c.c. glacial 

 acetic acid, and 300 c.c. distilled water (from Zeit. zviss. Mik., xi, 

 p. 378). Yak Beneden has used a saturated solution in 25 per 

 cent, acetic acid, and Lo Bianco {Mitth. Zool. Stat. Neapel, ix, 

 1890, p. 443) a mixture of 2 parts saturated solution with 1 part of 

 49 per cent, acetic acid. 



It is sometimes advisable to take the most concentrated solution 

 obtainable. For some very contractile forms (coral polypes, Planaria), 

 a concentrated solution in warm or even boiling water should be 

 employed. For Arthropoda alcoholic solutions are indicated. Delicate 

 objects, however, may require treatment with weak solutions. 



Objects should in all cases be removed from the fixing bath as 

 soon as fixed, that is, as soon as they are seen to have become 

 opaque throughout, which may be in a few minutes or even 

 seconds. 



Wash out with M^ater or alcohol. Alcohol is cdmost ahvays 

 preferable. Alcohol of about 70 per cent, may be taken, and 

 (Mayer, hitern. Moiiatsschr. Atiat. Phys., iv, 1887, p. 43) a little 

 tincture of iodine * may be added to the liquid, either alcohol or 

 water, used for washing, enough to make it of a good port-wine 

 colour, and the mixture be changed until it no longer becomes 

 discoloured by the objects. Apathy [Mikrotechnik, p. 148) takes 

 a 0-5 per cent, solution of iodine in strong alcohol, leaves the 

 objects in it (suspended) until they have become of about the 

 same colour as the solution, and then Avashes for twenty-four 

 hours in pure alcohol, 



J. Baker {op. cit.) states that the precipitate is probably either 

 mercurous chloride, or a phosphate formed by reaction with the 

 phosphates found in cells. 



In obstinate cases solution of iodine in iodide of potassium {e.g. 

 Lugol's) may be taken. IVLvyer {Zeit. zviss. Mik., xiv, 1897, p. 28) 

 makes it by dissolving 5 grm. of iodide of potassium in 5 c.c. of distilled 

 water and mixing this with a solution of 0-5 grm. of iodine in 45 c.c. of 

 90 per cent, alcohol, but seldom uses the mixture concentrated, merely 

 adding as nmch of it as is required to the alcohol or water containing 

 the objects. The important point is, that the iodine and iodide be 

 employed together. The iodine may be washed out in obstinate cases 

 with magnesia water. Similarly Apathy {Mitth. Zool. Stat. Neapel, xii, 

 1897, pp. 729, 730). 



It has been objected to this process that iodine in potassium iodide 

 precipitates corrosive sublimate instead of dissolving it. That is true, 

 but the precipitate is soluble in excess of the precipitant. 

 * Refer to, and contrast, Susa, § 93. 



