96 APPLIED BACTERIOLOGY 



maining about twelve hours, they are generally sufficiently 

 hardened. 



Imbedding. — After hardening, the tissue is imbedded, in 

 order to prepare it for the section - cutting machine. 

 The simplest method is to soak the pieces of tissue in a 

 strong solution of gum arable for a few minutes, after 

 which they are fixed to a cork. When dry or nearly dry, 

 the whole is immersed for some time in alcohol, which 

 abstracts the water from the gum, thus rendering the mass 

 sufficiently firm to be cut. The cork, with the pieces of 

 tissue firmly attached, is fixed in the clamp of the micro- 

 tome. Sections of the material are now cut from '02 to 

 ■05 mm. in thickness. Great care must be taken to keep 

 both the tissue and the knife-edge wet with alcohol. The 

 sections so cut are carefully transferred to alcohol by means 

 of a needle and brush. 



Imbedding in Paraffin. — This method, although seldom 

 employed, gives very good results. The material, after 

 hardening in absolute alcohol, is placed in a mixture of 

 equal parts of absolute alcohol and chloroform for twenty- 

 four hours, and finally for the same length of time in 

 pure chloroform. After this it is laid in paraffin dissolved 

 by heat in chloroform, and remains in this solution for 

 three hours at about 35° C. A paper mould or small card- 

 board box (such as a pill-box) is about one-third filled with 

 melted paraffin-wax ; the prepared pieces of tissue are laid 

 on the centre of the wax layer, then more melted paraffin- 

 wax is poured on in such a way as to enclose the material in 

 the centre of a small block of wax. When set, which may 

 be hastened by immersion in cold water, the block is 

 trimmed to a suitable form with a knife to fit the clamp of 

 the microtome. The sections are cut without any moisten- 

 ing fluid. 



The sections are transferred to xylol in order to dis- 



