82 IMBEDDING METHODS. 



by means of which a cylinder of paraffin containing the object may be 

 lifted out as soon as cool. 



WILSON (Zeit. wiss. Mik., xvii, 1900, p. 169) makes orientation lines 

 by imbedding alongside the objects strands of osmium -blackened nerve- 

 fibres. See also a further development by Wilson, ibid., xxvii, 1910, 

 pp. 228 and 231. 



143. Cooling the Mass. Whatever method of imbedding and 

 orientation in the molten paraffin has been employed, the important 

 point now to be attended to is that the paraffin be cooled rapidly. 

 The object of this is to prevent crystallisation of the paraffin (which 

 may happen if it be allowed to cool slowly) and to get as homogeneous 

 a mass as possible. 



If the definitive imbedding has been done in a watch-glass, hold 

 it on the top of cold water until all the paraffin has solidified, and 

 then let it sink to the bottom. When thoroughly cool, cut out 

 blocks containing the objects. If the watch-glass has been smeared 

 with a drop of a mixture of equal parts of glycerine and water before 

 putting the paraffin into it, the solidified paraffin will generally 

 detach itself in a single cake and float up in a few minutes, or hours 

 at any rate. Do not attempt to remove it entire by warming the 

 bottom of the watch-glass. Similarly with the paper trays or metal 

 imbedding boxes. Or you may put them to cool on a cold slab of 

 metal or stone. 



SELENKA cools the mass by passing a stream of cool water through 

 the imbedding tube described above ( 137). MAYER cools the mass in 

 the paraffin-tight moulds ( 137) by passing cold water through a special 

 movable water-bath, which allows of the arrangement of the objects by 

 transmitted light under a dissecting microscope, see Mitth. Zool. Stat. 

 Neapel, iv, 1883, p. 429 ; Intern. Monatssclir. Anat. Hist., iv, 1887, p. 39. 

 A complicated apparatus for the same purpose is described by MEISSNER 

 (Zeit. wiss. Mik., xviii, 1902, p. 286). Similarly, HAHN, ibid., xxv, 1908, 

 p. 184, and KAPPERS, ibid., xxiv, 1907, p. 254. See also FARKAS, ibid., 

 xxx, 1913, p. 168, for experiments on cooling methods. 



The paraffin blocks with the objects are now mounted on the 

 carrier of the microtome in position for cutting, and pared to the 

 proper shape (next ). If any bubbles or cavities or opaque spots 

 be present, prick with a heated needle till all is smooth and homo- 

 geneous. The same should be done if any cavities present them- 

 selves in the course of cutting. In bad cases, re-imbed. 



144. Shape and Orientation of the Block of Mass to be cut. These 

 differ accordingly as the cutting is done with a slanting knife or a 

 square-set knife (see next ). In the first case, the block is best 

 trimmed to a three-sidedjmsm, and orientated as in Fig. 4, so that 



