88 CHAPTER VIII, 



satiou of tlie paraffin (vvhicli may happen if it be allowed to 

 cool slowly) and to get as liomogeneous 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 

 pai-affin 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. 



Selbnka cools the mass by passing a stream of cool water tlirougli 

 tlie imbedding tube described above (§ 137). Maybe cools the mass in 

 the paraffin-tight moulds (§ 137) by passing cold water thi-ough a special 

 movable water-bath, which allows of the an-angement of the objects by 

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

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

 A complicated apparatus for the same purpose is described by Meissnee 

 (Zeit. wiss. Mile, xviii, 1902, p. 286). Similarly, Hahn, ibid., xxv, 1908, 

 p. 184, and Kappees, ibid., xxiv, 1907, p. 254. 



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 homogeneous. The same should 

 be done if any cavities present themselves in the course of 

 cutting. In bad cases, re-imbed. 



144. Shape and Orientation of the Block of Mass to be cnt. — 



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-sided prism, and 

 orientated as in Fig. 4, so that the knife enters it at the 

 angle a and leaves it at the angle c. When the section is 

 cut it will adhere to the knife only by the angle c, and can 

 thus most readily be removed by means of a brush or needle. 

 The object itself should come to lie in the block close to the 

 line b c, so that the knife at first cuts only paraffin, and 



