88 



HEWSON SWIFT 



alcohol or Navashin's fixation and consequently they are not suitable for 

 most sectioned material. Zinc chloride further causes a shift of the nucleic 

 acid peak to 270 m/u. Chloral hydrate also removes proteins after acetic 

 alcohol but not after formalin fixation. Absorption curves essentially free 

 from scatter can be obtained by the use of butyl methacrylate as an em- 

 bedding medium (polymerized by ultraviolet light without plasticizer). 

 Sections up to 5 /x in thickness may be cut with a glass knife, as in prepara- 

 tion of material for electron microscopy. The sections may be spread on 

 slides with 95% alcohol, and mounted in glycerin with the methacrylate 

 still present. Artifacts may be produced, however, by the methacrylate 

 during the process of polymerization. Also it may be hard to remount the 

 section if the methacrylate is once removed, so that sections cannot be 

 measured both before and after they are subjected to enzyme treatment. 



300 350 250 300 3SO 



Wavelength 

 Fig. 8. The effects of mounting media. Left: Frog melanophore nucleus fixed in 

 10% formalin, mounted in butyl methacrylate and sectioned at 5 p.. Section was first 

 measured with embedding medium in place (curve C) ; methacrylate was removed in 

 acetone -xylol, and the section was remounted in glycerin and the same area measured 

 (curve B). Curve A shows the same area measured in glycerin-water (1:1). The sub- 

 traction curves (.4 - B and B - C) show anomalous dispersion primarily of the 

 protein component. Right: Frog liver nucleus, prepared as above. A. In glycerin. B. 

 In methacrylate. Subtraction curve (A - B) shows both nucleic acid and protein 

 have contributed to nonspecific light loss. The effects are complicated by the slight 

 swelling of tissues in methacrylate. 



