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ZHOU MEI-YUN (Shanghai Institute of Cell Biology,Academia Sinica)  
Polyethylene glycols (PEG) are agroup of polymers with different mo-lecular weights (from 200 to 6000)and melting points depending on thedegree of polymerisation.They arewater-soluble and easily mixed withmost organic solvents;they are chemi-cally inert and do not dissolve lipidsand glycogen. Low molecular weight PEG beinga colourless,smell-less,and viscous fluidcan be mixed with water and Epoxyresin at room-temperature.It isconvenient to operate.The present work describes the use of PEG400 made in China as a dehydrationfluid in ultra-thin sectioning.Thepreservation of cellular ultrastruc-ture was found to be satisfactory. Kidney of mouse and gastrula ofCynops orientalis were cut into smallpieces and prefixed in 2.5% glutaral-dehyde buffered with 0.1 M sodiumcacodylate pH 7.4 for 1.5 hrs.Af-ter washing with 0.1 M sodium cacodylate buffer pH 7.4 about 3 hrsthe tissues were postfixed in 2% osmicacid for 1 hr.After washing thetissues were dehydrated with increasing grades of PEG 400 solution (from50% to 100%),3—6 hrs for eachgrade.The tissues were then infiltrated with fresh Epon 812 mixtureat room temperature for 48 hrs.Withthe aid of agitator the dehydrationand infiltration should be accomplished thoroughly.Polymerization wascarried out at 60℃ for 48 hrs. The micrographs showed that theultrastructure had been well preser-ved.Crystal lattices of yolk plateletsof gastrula cells of Cynops orientalis andsome small vesicles in the peripheralzone of the yolk were distinctly shownjust as that prepared with the silyla-ted aminoplastic-water soluble embedding medium. According to the property of lowmolecular weight PEG,it could beexpected that this method would beuseful to the samples which should beavoided in contact with organic solvents such as alcohol and acetone.This method needs further investiga-tion.
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