so 
IOWA ACADEMY OF SCIENCES. 
commercial formalin 10^ 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 allowed considerable lati- 
tude in the time of hardening. 
Living embryos in which 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 slowly 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. 
