LAMELLAR SYSTEMS 121 



liquid nitrogen mixtures (Fernandez-Moran, 1959c). Attainment of 

 these extremely low temperatures, at which all other substances 

 solidify, and at which diffusion or recombination of unstable chemi- 

 cal species within the tissue matrix can be practically eliminated, is 

 therefore the main reason for the use of liquid helium II in con- 

 nection with ultrastructure studies. Although better preservation of 

 fine structure has already been achieved at this preliminary stage, 

 it must be emphasized that the full benefits of this approach are to 

 be expected only if the trapped free radicals can be effectively im- 

 mobilized and made electron-optically visible within the intact 

 tissues by suitable interaction with heavy atoms and (addition) 

 polymerization at lower temperatures. Liquid helium is chemically 

 inert and far safer to handle than isopentane-propane-liquid nitro- 

 gen mixtures or liquid hydrogen. Moreover, the widespread use of 

 liquid helium in industrial cryogenics research now makes it more 

 readily available, and the practical difficulties connectied with its 

 application can be easily solved once the basic experimental tech- 

 niques have been mastered. 



In collaboration with Professor Samuel C. Collins and his asso- 

 ciates at the Cryogenic Engineering Laboratory of the Massachu- 

 setts Institute of Technology, a series of preliminary experiments 

 with liquid helium II have been carried out during the past year. 

 As shown in Fig. 1, the basic equipment consists of a specimen stage 

 attached to a shielded helium Dewar which can be evacuated with 

 a large forepump of 311 cu. ft./min. capacity. For processing even 

 a limited number of specimens it is necessary to prepare relatively 

 large amounts of bulk liquid helium II because of its small heat of 

 vaporization. This is accomplished by filling the inner Dewar with 

 liquid helium I and rapidly lowering the vapor pressure to approxi- 

 mately lOfi, equivalent to —272.2° C. The simple type of stage 

 consisting essentially of a large glass stopcock makes it possible to 

 introduce fresh or glycerinated tissues into the liquid hehum II bath 

 without breaking the vacuum (Fernandez-Moran, 1959d). High- 

 speed photographs, recorded in collaboration with D. Eldridge of 

 the Electrical Engineering Department of M. I. T., show that im- 

 mersion of the specimen into the bulk liquid does not produce boil- 

 ing (Figs. 2, 3), suggesting that equilibration of the temperature 

 difference takes place very rapidly. If necessary, the specimen can 

 be precooled with propane-liquid nitrogen or liquid hydrogen and 

 immediately plunged into liquid helium II to prevent transitional 



