218 EARL W. FLOSDORF 



globin {22). Further details will be found in Section E5 on the in- 

 fluence of oxygen on the life of frozen-dried preparations. 



5. Histological and Cytological Preparations 



Freeze-drying has been employed extensively for many years by 

 histologists and cytologists. The technique as practiced is quite 

 different from the method used today for biologicals, pharmaceuticals, 

 microorganisms, and other products discussed. The procedure is 

 much slower and the products are maintained at lower temperatures 

 without heating. It is for these reasons, as well as the fact that tis- 

 sues are generally more difficult to dry than liquids, that the process is 

 more time consuming and requires more elaborate equipment. 



It was nearly sixty years ago that Altmann {17) stated that biologi- 

 cal materials could be dried without shrinkage at a temperature of 

 — 20°C. This was published in connection with his cytological work 

 on bioblasts. Forty years later Gersh {1^2) described equipment for 

 fixation that involved the use of liquid ammonia to provide the low- 

 temperature and a diffusion pump and phosphorus pentoxide for 

 removal of the moisture. With this equipment Gersh was able to 

 confirrn the value of the procedure for fixation of some organs and 

 tissues (skin, cartilage, smooth muscle, liver, pancreas). Gersh ap- 

 plied the technique successfully to the investigation of the excretion of 

 uric acid and ferricyanide, the chemical nature of intracellular con- 

 stituents, and of the intracellular distribution of glycogen. Gersh 

 also used the method for the preservation of vitally, or supravitally, 

 stained preparations that fade in fixation or are incapable of satis- 

 factory preservation by other means. He found that fixation of or- 

 gans other than those just mentioned was less satisfactory and that 

 central nervous system material is exceedingly poorly fixed. Results 

 of very interesting investigations on these subjects were then pub- 

 lished by Bensley, Gersh, Stieglitz, Hoerr, and others during the next 

 three years {43). Goodspeed, Uber, and Avery reported on interest- 

 ing applications in plant cytology. They studied particularly the 

 character of fixation and reported on chromosome structure in Lilium 

 longiflorum (44)- 



6. Commercial Processing of Heat- Sensitive Materials 



During the past few years, freeze-drying has been applied to the 

 production of many medicinal products on a scale that fifteen years 



