22 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 119 



mended that this valve be opened while the piunp is still running to 

 prevent atmospheric ah' from forcing the pump to run backward or 

 forcing oil from the pump into the condensing chamber. 



Figure 8 illustrates assembly of the modified apparatus using a 

 refrigerated condenser. 



Preparation of Specimens 



The freeze-dry process will not improve a poor specimen. If a 

 specimen is emaciated, slightly deteriorated, or otherwise inferior 

 when it enters the chamber, it will be in no better condition when it 

 leaves. Sagged tissue, however, can often be reshaped ^^dth subcu- 

 taneous injections of water, and sagged body cavities can be restored 

 with cotton before freezing. 



The art and skill of the taxidermist will help decide the success or 

 failure of the freeze-dry technique. Many of the same tools and 

 methods of conventional taxidermy can be employed advantageously 

 in the freeze-dry process. Wires, supports, and other tricks of the 

 trade are useful. Also rapid freezing with liquid nitrogen or freezing 

 mixtures will hold a specimen in position. 



Initial freezing of the specimen. — Of all compounds in animal 

 tissue, water is the most abundant. For the average, it constitutes 

 70 percent of the animal's total weight. Water is found in cellular 

 and vascular spaces, and, in small quanties, in protein and carbohy- 

 drates. Of the total liquid content, 20 percent is usually in extracel- 

 lular fluid ; approximately 25 percent of this extracellular fluid is plasma, 

 and the remainder is interstitial fluid, mostly water. About 76 per- 

 cent of muscle tissue is water. The Ro^\^ltree data (see Harrow, 1951) 

 relating to the biochemistry of man is a good general guide to the 

 distribution of tissue water in most mammals. 



Water in tissue is found almost always in combination ^^^th naturaUy 

 occurring salts; for this reason, freezing should be brought about in 

 the shortest possible time. Slow freezing invariably leads to the 

 formation of eutectics through the concentration of salts. As freezing 

 water separates itself from a solution by the process of ion diffusion, the 

 salts become more highly concentrated than they were in the original 

 solution. 



Eutectics have lower freezing pomts and lower vapor pressures than 

 water; thus, their formation during the process slows do^\^l drying. 

 Due to unfrozen saturated fluid in the tissue and resulting surface 

 tension, shrinkage occurs. Rapid freezing, which reduces the 

 formation of eutectics, can be accomplished by using the freezing 

 mixtures listed in table 6, liquid nitrogen, or a freezing chest with a 



