180 IMBEDDING METHODS. 



once you will probably get cavities filled with water formed 

 in your paraffin. Or you may put them to cool on a block of 

 cold metal or stone. 



SELENKA recommends cooling the mass by passing a stream 

 of cold water through the imbedding tube described above 

 ( 273). 



The objects having been mounted on the carrier of the 

 microtome in position for cutting, pare the blocks square to 

 the knife, and sufficiently close down to the objects, and go 

 round them with a lens. If any bubbles or cavities or opaque 

 spots be present, prick with a heated needle till all is smooth 

 and homogeneous. Minutes spent in this way are well in- 

 vested. 



It is well to cut within a few hours of imbedding if the 

 structures be at all delicate, as paraffin may continue to 

 crystallise slowly to a certain extent even after rapid cooling. 

 But this danger is very greatly diminished if the mass have 

 been properly cooled. 



280. Cutting and Section-stretching. Paraffin sections are 

 cut dry that is, with a knife not moistened with alcohol or 

 other liquid. By this means better sections are obtained, but 

 a difficulty generally arises owing to the tendency of sections 

 so cut to curl up on the blade of the knife. It is sometimes 

 difficult by any means to unroll a thin section that has curled. 

 To prevent sections from rolling, the following points should 

 be attended to : 



First and foremost, the paraffin must not be too hard, but 

 must be taken of a melting-point suitable to the temperature 

 of the laboratory (for the winter season the temperature of 

 the laboratory being between 15 and 17 C., a paraffin 

 melting at about 45 C. should be taken ; for hot summer 

 weather, laboratory at 22 C., a paraffin of 48 C. melting- 

 point). 



The exact degree of hardness necessary must be determined 

 by experiment. If, after cutting has begun, the paraffin be 

 found to be too hard, it may be softened by the following 

 simple expedient (Foi, Lehrbuch d. vergl. mikr. Anat., p. 123) : 

 A lamp provided with a parabolic reflector is set up near the 

 microtome in such a position that the heat-rays of the flame 

 are thrown by the reflector on to the imbedded object. The 



