SECTION CUTTING 105 



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 

 an ether-freezing microtome, such as Swift's (Fig. 20) or 

 Cathcart's. A microtome in which the freezing is 

 effected by carbonic acid is now frequently employed 



FIG. 20. Swift's ether-freezing microtome. 



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 trans- 

 ferred successively to two or three lots of distilled water, 



