FREEZING-DRYING PREPARATION OF TISSUE 5 



wholly satisfactory, Packer and Scott (1942) developed a cryostat 

 of a new design that is the finest instrument yet devised for the 

 freezing-drying of tissues. An important feature of this apparatus 

 is that the frozen and thoroughly dried tissue can be brought 

 gradually to the temperature of the melted paraffin, and then it 

 can be embedded without contact with the moisture of the air. 

 Previous practice was to transfer the very cold tissue from the 

 cryostat to the air, and then plunge it directly into melted paraflEin, 

 thus subjecting it to a sudden temperature change of about 100°. 

 Sjostrand (1944) described a freezing-drying apparatus somewhat 

 simpler than the Packer-Scott instrument but it was not designed to 

 permit paraffin infiltration within the apparatus. 



PACKER-SCOTT METHOD FOR FREEZING- 

 DRYING TISSUES 



The Freezing-Drying Apparatus. In the diagram of the ap- 

 paratus, which is made of Pyrex glass (Fig. 1), the drying chamber 

 (C) is a 2.5 in. tube 12.5 in. long surrounded by a jacket of about 

 3.5 in. diameter that can be exhausted through stopcock B connected 

 by glass tubing to S. The 3 gal. Pyrex Dewar flask (Di), containing 

 solid carbon dioxide in butyl alcohol is used to cool the drying 

 chamber, and it is arranged so that it can be easily lowered away 

 from the apparatus. A commercially built refrigerator has also been 

 employed in place of solid carbon dioxide for the cooling by Hoerr 

 and Scott (1944). When a pressure of 1 mm. of mercury, or less, 

 is maintained in the space E, and paraffin is in tube C, the equi- 

 librium temperature over the paraffin is about — 66°. As used at 

 present, there is no occasion to employ temperatures higher than 

 — 66°, but Packer and Scott point out that a thermocouple sealed 

 in space E could be used to operate a thermostat which in turn could 

 control the current in the paraffin heater (D) in order to maintain 

 temperatures above — 66°. The heater (D) is required to melt the 

 paraffin in tube C so that the tissues held in the copper gauze basket 

 (Ci) can be embedded in vacuo. 



The heater (D) is constructed by covering a thin-walled copper 

 cylinder with liquid porcelain (Insa-lute), and, after dry, winding 

 No. 18 Nichrome wire (about 70 ohms) over it and applying another 

 layer of liquid porcelain over the wire. A thin-walled sleeve of cop- 



