PARAFFIN METHOD 85 



cooled rapidly. The object of this is to prevent crystalHsation 

 of the paraffin (which may happen if it be allowed to cool slowly) 

 and to get as homogeneous a mass as possible. 



If the definite 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 glycerin 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 (§ 156). Mayer cools the mass in 

 the paraffin-tight moulds (§ 156) 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. Monatsschr. Anal. Hist., iv, 1887, p. 39. 

 A complicated apparatus for the same purpose is described by INIeissner 

 {Zeit. iviss. Mik., xviii, 1902, p. 286). Similarly, Hahn, ibid., xxv, 1908, 

 p. 184, and K.\ppers, 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 

 homogeneous. The same should be done if any cavities present 

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



164. 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-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 h c, so that the knife at first 

 cuts only paraffin, and that if the section begins to roll it may be 

 caught and held down by a brush or section-stretcher before the 

 object itself is reached. For the square-set knife the block is best 

 trimmed to a four-sided prism, and orientated as in the first case, 

 so that the knife first touches one angle, if only isolated sections 

 are to be cut. But if ribbons (§ 174) are to be cut, the block must 

 be orientated with one of its sides parallel to the knife-edge, and 

 the opposite side must be strictly parallel to this one. 



