266 TECHNIQUE 



tration is less rapid than acetic acid. It causes some shrinkage but 

 does not harden tissues very noticeably. 



ForiJialin .—Weak solutions (10 per cent) of formalin in water 

 are used as fixing agents. They cause changes in protein by form- 

 ing additive com}:)oinids. The action is slow and the tissues are 

 hardened due to eflfects on the cell membrane. Shrinkage is not an 

 immediate result but may follow during later stages in technique. 



Alcohol.— This precipitates proteins, the precipitates of nucleo- 

 proteins being soluble in water. It dissolves lipoids but precipitates 

 glycogen. Tissues treated with alcohol shrink and harden and later 

 staining is difficult. 



Osmium 7V^/-o.i-i(^g.— Solutions of this chemical in water are 

 called osmic acid. This fixes and blackens fat, chondriosomes, and 

 the Golgi apparatus. Its effect upon the remainder of the proto- 

 plasm is one of fixation without precipitation. Shrinkage is slight, 

 penetration is slow and variable, and staining is made difficult. 



Chromic ^cid. — Solutions of this reagent act as oxidizing agents. 

 It precipitates proteins presumably by forming compounds with 

 them, but leaves fat and lipoids unaflfected. It should be washed 

 out with water, so that interfering precipitates are not formed, as 

 they would be if washed with alcohol. The chromic acid solution 

 causes slight shrinkage and hardening but makes staining with 

 basic dyes easier. 



Potassium Z)w7?/'o?/?a/f. — Unacidified solutions of this reagent 

 render albumin insoluble but do not precipitate it. It is similar 

 to osmium tetroxide in that it renders proteins insoluble without 

 precipitating them. Chromatin is dissolved, so that it is a poor 

 chromatin fixative. Its action is slow, and tissues should be washed 

 in water after its use. When made acidic, its solutions fix in the 

 manner of chromic acid. 



Mercuric Chloride. — Satursited solutions of this reagent in water 

 act as precipitants of proteins, have no effect on lipoids, and do 

 not destroy chondriosomes. The solution penetrates quickly and 

 causes some shrinkage. It is necessary to wash it out after short 

 fixation. Crystals tend to form and should be dissolved later by 

 washing in alcohol to which iodine has been added. 



These are but a few of the numerous reagents in use, and only a 

 brief note is made of their action. For example, it was just stated 

 that acetic acid and picric acid are both useful, but that the first 

 causes swelling of tissue and the second causes shrinkage. By a 

 proper combination these effects may be counterbalanced. A review 



