§0 IOWA ACADEMY OF SCIENCES. 



commercial formalin lOi per cent increase; in the chromo- 

 acetic-formalin mixture no change in diameter. 



In fixing mammalian tissues chromo-acetic-formalin 

 gives good results, in some tissues better results than I have 

 been able to obtain with any other fixing fluid. It seems 

 to be especially good for glands and mucous epithelium. 

 I have not obtained satisfactory results in treating nerv- 

 ous tissue with it. In the fifth edition of the Vade-Mecum 

 Lee suppresses the formula for this fixing fluid, and con- 

 cludes that his previous good results with it were acci- 

 dental. I cannot agree with him. 



In two or three days after being prepared the fluid 

 changes from the chromic acid brown color to a chrome 

 alum blue, due to the reduction of the chromic acid to 

 chromic oxide. At the same time some of the formalin is 

 oxidized to formic acid. Before these changes take place 

 I find that the fluid has the characteristic defects of 

 chromic acid; that is, the fixation and general preserva- 

 tion is not uniform, and staining is rather difficult. After 

 six months standing the fluid does not appear to deteriorate. 



The length of time required for fixation depends upon 

 the size of the object. I have not observed any indication 

 of bad results after treatment- of 24-48 hours. Fixation 

 should be followed by thorough washing in water, but the 

 latter process is not necessarily so prolonged as when 

 chromic acid alone is used. The formalin itself being 

 such a good preservative one is allow^ed considerable lati- 

 tude in the time of hardening. 



Living embryos in w^iich the muscular system is far 

 enough developed te be functional are likely to become 

 distorted when killed in the chromo-acetic-formalin. 

 My custom has been to allow them to die slowdy in 

 chromo-acetic acid (chromic acid 0.2 per cent, acetic acid 

 0.1 per cent). This they do retaining their natural shapes. 

 As soon as dead they are transferred to the chromo-acetic- 

 formalin mixture. 



