42 CHAPTER IV. 



servation that the reagent must be properly used, and not 

 applied to objects for which it is not fitted. For instance, 

 its power of penetration is extremely bad ; it will not fix 

 properly, even in a loose-celled tissue, through more than a 

 layer of about five cells thick. It is therefore suitable only 

 for very small objects or for very small pieces of tissue, such 

 as suffice for cytological or histological work. But it is not 

 suitable at all for voluminous objects, such as the organo- 

 logical anatomist and the embryologist so frequently have to 

 do with. It has not the character of a general reagent. 



In previous editions I have quoted some authors who have 

 accused this reagent of faulty preservation. It must be 

 admitted that it has the defect of easily causing over-fixation 

 of superficial cells, and, owing to its defective penetration, 

 insufficient fixation of deep-lying ones. It requires to be 

 used in the proper manner, and, above all, with the proper 

 objects ; and it is by no means of such universal applicability 

 as has been supposed. 



It may be used for prolonged hardening, e. g. of nervous 

 tissue, and is a very good reagent for the purpose. 



47. Chromo-aceto-osmic Acid (FLEMMING, Zeit. f. iciss. Mik., 

 1, 1884, p. 349), SECOND or STRONG formula : 



1 per cent, chromic acid . .15 parts. 



2 per cent, osmic acid . . 4 ,, 

 Glacial acetic acid ... 1 part. 



If 2 per cent, osmic acid solution should not be at hand, you may conve- 

 niently make the mixture by taking 



10 per cent, chromic acid . . . .15 parts. 

 1 per cent, osmic acid . . . . . 80 ,, 

 Glacial acetic acid . . . . .' . 10 

 Water 95 



If this mixture be kept in stock in large quantities, it 

 may go bad, probably on account of the large proportion of 

 organic acid contained in it. I therefore recommend that it 

 be made up from time to time from stock solutions, in which 

 the osmic acid is kept separate from the acetic acid; the 

 proportions being as follows : 



CrO s 0-15 



Os0 4 .... . 0-08 



Acid. acet. . . . . .1*00 



Aq ' 19-00 



