SECTION CUTTING 89 



In this gum solution the pieces remain for twelve to 

 forty- eight hours, according to their size and the time 

 at the disposal of the investigator, and are then cut on 

 one of the numerous ether-freezing microtomes now to be 

 obtained, such as Swift's (Fig. 18) or Cathcart's. A 



FIG. 18. Swift's ether-freezing microtome. 



microtome in which the freezing is effected by carbonic 

 acid is now frequently employed and acts well. Liquid 

 carbonic acid, contained in a cylinder, sprays by its own 

 pressure on to the under surface of the plate on which the 

 block of tissue rests ; the tissue quickly freezes and is 

 then cut. This form of microtome works satisfactorily in 

 the hottest weather. The material must not be frozen so 

 hard that the sections roll up and fall off the knife ; the 

 sugar in the above solution should prevent this. The 

 sections are transferred successively to two or three lots 



