56 AIDS TO BACTERIOLOGY 



certain disadvantages the cellular structure may be dis- 

 torted, the sections are not very thin or regular, and 

 delicate tissues are apt to be torn. 



Embedding in Paraffin. This method gives the thinnest 

 sections and preserves the structures. The material, 

 after hardening, is placed in absolute alcohol for twelve 

 to twenty-four hours, and then in pure xylol until it 

 looks clear. Chloroform may be used instead of xylol. 

 After this it is placed in a bath of melted paraffin wax, 

 in which it remains for six to eighteen hours, according 

 to the size, until thoroughly impregnated with the melted 

 paraffin. The paraffin is kept in the melted state in a 

 special bath or hot- water oven, the temperature of which 

 is regulated by a thermo-regulator: Hearsons make a 

 special form with ' Excelsior ' gas regulator. As regards 

 the paraffin wax to be used, opinions differ, but probably 

 one having a fairly high melting-point e.g., 50 to 52 C. 

 is the best all round. Some tissues, such as skin, must 

 not remain longer than is absolutely necessary in the 

 melted paraffin, or they become hard and friable, and 

 will not cut. After impregnation the material has to be 

 embedded; a paper mould or small cardboard 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 is preferably 

 hastened by immersion in cold water, the block is trimmed 

 to a suitable form with a knife, and cemented to the 

 carrier of the microtome by softening the base of the 

 mass with a match-flame, and melting the margins with 

 a hot wire. The sections are cut without any moistening 

 fluid. 



The sections are next mounted on slides. This is done 

 by placing them in a dish of warm water (a pastry- tin 

 does well, and may be warmed over the Bunsen), the 

 temperature being so adjusted that the paraffin becomes 

 softened, but not melted, so that the sections become flat. 

 A slide is introduced into the water under a section, which 

 is then lifted up on it, being fixed and adjusted by means 

 of a needle. The water is then tilted off, and the specimen 

 allowed to dry for not less than two to three hours in 

 the warm incubator. If the sections be thin, they adhere 



