10 



Ground Sections 



General Principles 



Nature of the Process 



Previous chapters have dealt with 

 the preparation of whole specimens 

 either mounted individually, smeared, or 

 squashed on a slide. There are many speci- 

 mens which, in order that their micro- 

 scopic structure may be examined, have 

 to be cut into thin structures. The more 

 usual methods of cutting such materials 

 are given in this and the next four chap- 

 ters. The present chapter, which describes 

 the preparation of sections of materials 

 too hard to be cut by anj^ conventional 

 method, had better be ignored by anyone 

 not specifically interested in this process. 

 Sections of hard materials such as bone, 

 the calcareous skeletons of coral, and even 

 some of the hardest vegetable materials, 

 must be prepared in two stages. The first 

 of these stages is the preparation of a 

 crude section from a half to one millimeter 

 in thickness, while the second stage con- 

 sists in grinding this down while leaving 

 both sides polished. 



Preparation of the Crude Section 



It is presumed that we are dealing with 

 material which cannot be cut with a knife 

 and must, therefore, be cut with a saw. 

 Most woodcutting saws are not hard 

 enough for the purpose and the choice lies 

 between the ordinary hacksaw, intended 

 for cutting metal, and a jewelers' saw. 

 The disadvantage of a hacksaAV is that it 

 is very coarse, so that only thick sections 

 may be cut; the disadvantage of the 

 jewelers' saw is that, unless it is guided by 

 an expert hand, it will not cut a parallel- 

 sided section. Whichever saw is selected, 

 however, it should have relatively coarse 



teeth, particularly if bone, or material 

 containing very much animal matter, or 

 material which has been embedded in 

 resin, is to be cut. These materials choke 

 the teeth of a fine saw; the tooth marks 

 left by a coarse saw do not matter, for 

 they will be ground and pohshed out. For 

 very hard substances, such as teeth, it is 

 often necessary to use a circular diamond 

 saw, which is usually available in depart- 

 ments of geology Avhere rock sections are 

 cut. Even teeth, however, may be cut 

 with a jewelers' saw provided that the 

 blade be changed at intervals and that the 

 entire operation be conducted under the 

 surface of water. 



Another type of preparation is that in 

 which it is desired to preserve both the 

 hard and the soft portions at the same 

 time. A standard example of this is the 

 preparation of a coral, of which it may be 

 desired not only to section the calcareous 

 skeleton but also to retain in position the 

 soft parts of the animal within. This can 

 only be done by embedding the material in 

 some substance nearly as hard as the 

 skeleton itself. A number of resins have 

 been proposed for this purpose. The au- 

 thor always prefers, however, to use 

 Canada balsam because, though it is 

 gummy in the final grinding, it has the ad- 

 vantage that it need not be removed for 

 mounting, and thus obviates one rather 

 laborious stage of the process. As an 

 alternative one may employ the process 

 of Henrichi 1916 (4349, 6:45) in wliich 

 gum damar is substituted for balsam, 

 though the method which is described in- 

 volves the use of machinery not normally 

 available in laboratories. The technique of 



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