LAMELLAR SYSTEMS 



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Fig. 2. Liquid helium II (arrow) in shielded Dewar at T" K, showing charac- 

 teristic quiescent appearance. Because of the exceedingly high heat conduc- 

 tivity of helium II, a temperature difference between top and bottom o* the 

 liquid sufficiently great to allow the formation of vapor bubbles cannot be 

 produced. 



Fig. 3. Serial high-speed photographs showing immersion of specimen in 

 liquid helium II without producing boiling. (Courfesy, Professor Harold E. 

 Edgerton, Department of Electrical Engineering, M. I. T.) 



Technical Requirements and Equipment for Cryofixation. 

 In order to avoid ice-crystal and extraction artifacts during the 

 processing cycle, which may take several weeks in certain specimens, 

 a special "cryofixation assembly" (Fig. 4) was developed for semi- 

 automatic freeze-substitution and photopolymerization embedding 

 within a modified Harris refrigeration unit designed to maintain 

 — 100° C. The sealed specimen chamber (Fig. 5), formed by a 

 Teflon gasket bounded by thin Plexiglas sheets and sandwiched 

 between aluminum plates, permits regulated fluid transfer through 

 inserted valves. This type of chamber is also useful for other types of 

 preparations, since whole tissues can be embedded in one block and 

 the segments removed without distortion for precise sectioning in 

 any desired orientation. All substitution and embedding fluids were 



