4 MICROSCOPIC TECHNIQUES 



tissue, and the absence of cell shrinkage. These are no inconsider- 

 able advantages, and the freezing-drying technique should be given 

 the preference wherever possible. 



Scott ( 1943) has been careful to point out that, while distortion 

 of mineral distribution might be expected to occur as the result of 

 ice crystal formation during the freezing and that artifacts might 

 be occasioned by the paraffin infiltration, neither of these appears to 

 be a serious difficulty in the more recent improved techniques. As 

 Scott indicated, on the one hand ice crystal formation could be 

 readily recognized should it occur in a manner that might influence 

 interpretation, and on the other it has been impossible thus far to 

 find evidence of distortion resulting from the infiltration, although 

 various control experiments have been performed to test this 

 possibility. 



Gersh (1932) extended the Altmann method of dehydrating tissue 

 in vacuo at liquid-air temperatures, and the improved procedure is 

 known as the Altmann-Gersh technique.* In a critical study Scott 

 (1933a) pointed out that the dehydration temperature of —20° 

 used by Gersh was not low enough to prevent a certain amount of 

 ion diffusion since this temperature is above the eutectic point of 

 certain naturally occurring salt systems. In the improved cryostat 

 of Packer and Scott (1942), to be described in detail later, the 

 temperature is maintained below — 54.9°, the eutectic point of 

 CaCl2.6H20. When he first indicated the desirability of using lower 

 temperatures, Scott ( 1933a) recommended the use of alcohol cooled 

 to — 177°, instead of liquid air, since the latter gives rise to a gas 

 envelope around the tissue which retards the rate of freezing. A 

 further improvement was effected by Hoerr ( 1936) , who found that 

 more rapid freezing (hence smaller ice crystals) was obtained by 

 placing tissue in isopentane cooled to — 160° to — 195° by means of 

 liquid nitrogen. Among others, Simpson (1941) confirmed the 

 advantages of the isopentane method and, in addition, pointed out 

 the desirability of employing small pieces of tissue for treatment 

 since the centers of larger pieces do not yield sections of the highest 

 quality. 



After the appearance of the Gersh (1932) vacuum dehydrator, 

 other types were described by Goodspeed and Uber (1934) and 

 Scott and Williams (1936). However, since none of these was 



* The Gersh apparatus is available from A. S. Aloe & Co. 



