186 



METHODS AND FORMULAS 



FOl 



large-yolked eggs. Under these circum- 

 stances one is forced to fall back on the 

 ingredients themselves, which are sur- 

 prisingly few in number and which may 

 be conveniently classified into groups ac- 

 cording to the presumptive role which 

 they play in fixation. The first of these 

 great groups contains mostly metalhc 

 salts, presumably intended to denature, 

 or to form compounds with, the proto- 

 plasm. One or more of these ingredients 

 is found in almost all fixative solutions, 

 and they are, therefore, referred to in this 

 classification as 'primary fixative agents. 



PRIMARY FIXATIVE AGENTS 



1. "Osmic acid" (osmium tetroxide) 



2. Platinic chloride 



3. Mercuric chloride 



4. Cupric sulfate, nitrate, or chloride 



5. "Picric acid" (2, 4, 6 trinitrophenol) 



6. "Chromic acid" (chromium trioxide) 



7. Potassium or sodium dichromate 



8. Other inorganic salts 



9. Other organic reagents 



Since no one has yet proposed, nor is it 

 to be hoped that anyone will propose, a 

 mixture containing more than four of these 

 agents, it is possible to indicate mixtures of 

 them by using four numbers from the list 

 above. Thus, 1000 represents a fixative 

 containing only osmic acid, 1300, a fixative 

 combining osmic with mercuric, 1360, an 

 osmic-mercuric-chromic mixture, and so on. 



These symbols are always written in 

 their proper numerical order. Thus the 

 combination 3600 would indicate a mer- 

 curic-chromic mixture without any other 

 primary agent. The addition of osmic 

 would produce the symbol 1360. The 

 numerical rank assigned to these varying 

 agents has been designed, as far as pos- 

 sible, to render this classification easy to 

 handle. 



The vast majority of fixatives are modi- 

 fied by the addition of one, but never more 

 than two, fixative modifiers, which is the 

 term here used to describe the commonly 

 employed aldehydes and ketones. These 

 are incorporated in the symbohc classifica- 

 tion of a fixative by placing a decimal 

 point after the symbol for the primary 

 fixative agents and employing the first 



two positions to the right of this decimal 

 point for the fixative modifiers. The fol- 

 lowing list of fixative modifiers appear at 

 present to be sufficient: 



FIXATIVE MODIFIERS 



(.) 1. formaldehyde 



(.) 2. acetaldehyde 



(.) 3. acetone 



(.) 4. all other fixative modifiers 



The only other common additions to 

 fixative solutions are acids, of which never 

 more than two appear to be employed in 

 any one composition. It is, therefore, pos- 

 sible to indicate the acids as two further 

 figures placed to the right of the modifier. 

 For this purpose the fixative acids may be 

 listed as: 



FIXATIVE ACIDS 



The combination of the agents with the 

 modifiers and acids thus gives an eight- 

 figure description which will indicate the 

 composition of the fixative. This will be 

 rendered clearer by a few further ex- 

 amples. Thus: 0000.1000, formaldehyde 

 without other admixture; 0000.1010, for- 

 maldehyde-acetic; 0000.1012, formalde- 

 hyde-acetic-trichloroacetic; 1670.0010, os- 

 mic-chromic-dichromate-acetic. • 



The advantage of this method of classi- 

 fication is that it permits an indefinite ex- 

 pansion as new formulas appear in the 

 hterature. It also brings logically together 

 formulas of similar composition, but of 

 gradually increasing complexity. That 

 some system of classification is necessary, 

 is indicated by the fact that more than 

 300 classes of fixatives are at present in 

 existence. 



Additional difficulties are provided by 

 two-solution formulas: that is, such a 

 formula as those in which osmic acid and 



