22 FIXING AND HARDENING 



For routine vertebrate histological work Zenker, Susa and 

 Helly's Zenker-formol are indicated. 



We 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 diluted 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. 



33. Validity of Fixation Image. In the problem of fixation it is 

 necessary to notice that two criteria may be taken, firstly, the 

 condition of the ground cytoplasm and nucleus ; secondly, the 

 condition of the various categories of formed bodies, such as 

 mitochondria fat, Golgi apparatus, paraglycogen and so on. 

 The condition of the ground cytoplasm is usually of little interest 

 to those working in the formed bodies in the protoplasm, though 

 it is generally recognised that only the smooth appearance got 

 by osmium and chrome-osmium fixation resembles the condition 

 intra vitam. Thus the work of Fischer and Hardy is of little 

 importance to those working on the cytoplasmic inclusions, because 

 the cytoplasmic inclusions are not usually properly demonstrated 

 by fixatives which cause intense coagulation and production of arti- 

 facts in the ground cytoplasm. The condition of the ground 

 cytoplasm may, however, be quite different with two really good 

 methods for demonstrating the cytoplasmic inclusions, for example, 

 with the Weigl and the Nassanow methods, the former containing 

 corrosive sublimate, the latter chrome salts. That the methods 

 used to demonstrate the cytoplasmic inclusions do not merely 

 produce artifacts as claimed by Walker {Proc. Roy. Sac. B., 

 1927) can be shown by watching the fixation of cells under the 

 microscope, and by comparing the finished preparations with the 

 living cells. This may be done easily in tissue cultures, or with 

 smears of insects or molluscan gonads. It must be admitted, 

 however, that in the finished slides such fixatives as Bouin's 

 fluid (§ 115), more so alcoholic Bouin or acetic corrosive 

 sublimate (§ 68), produce preparations of cytoplasm which 

 show little resemblance to the cell or organism in the living 

 condition. It is admitted widely nowadays that osmic and 

 chrome-osmic fixatives are the best cytoplasm fixatives known 

 to us. 



In the case of chromatin it may be pointed out that, in the 

 living state, the intact nucleus is optically empty, except for the 

 nucleolus, and that the structures which we see after fixation may 



