74 IMBEDDING 



easily get with paraffin much thinner sections than you can with 

 celloidin ; and if you try to cut in paraffin objects of still greater 

 size, say an inch and upwards, it will frequently happen that you 

 will not get perfect sections at all, blocks of paraffin of this size 

 having a tendency to split under the impact of the knife. This 

 defect is, however, much reduced by the employment of a softer 

 paraffin than is usual. In this way Strasser {Zeit. wiss. Mik., 

 ix, 1892, p. 7) has obtained series of frontal sections 30 /x thick 

 through the entire human brain, in paraffin blocks measuring 

 10 X 15 cm. And Mayer, with the Tetrander microtome, has 

 obtained series of only 7-5 /x with a surface of 4^ x 3 cm. 



For very large objects celloidin is safer, because it does not 

 split, and presents advantages for the manipulation of the sections 

 obtained. For all classes of objects it has the advantages of 

 affording a transparent mass (which facilitates orientation of the 

 object) and of producing less shrinkage than paraffin (paraffin 

 unavoidably shrinks on cooling to at least 12 per cent.). It is 

 for these two reasons that celloidin is so frequently preferred by 

 embryologists — even for small objects. 



Aqueous masses, such as gum or gelatin, may render great 

 service in cases in which it is desired to avoid dehydrating tissues, 

 and to apply chemical tests to them. 



The lahoratonj worker should note the methijl benzoate (§ 131), 

 the dioxan (§ 130), the n-hutijl alcohol (§ 127), and the ceresin wax 

 methods (§177), as newer techniques which may prove helpful. 

 Attention may he called /o § 157 for recently used imbedding mani- 

 pulations. 



156. Imbedding Manipulations. Imbedding in a melted mass, 

 such as paraffin, is performed in one of the following ways. A 

 little tray or box or thimble is made out of paper, some melted 

 mass is poured into it, and the object placed in the midst of it. 

 Or, the paper tray being placed on cork, the object may be fixed 

 in position in it whilst empty by means of pins and the tray filled 

 with melted mass at one pour. The pins are removed when the 

 mass is cold. 



In either case, when the mass is cold the paper is removed from 

 it before cutting. 



To make paper trays proceed as follows. Take a piece of stout 

 paper or thin cardboard, of the shape of the annexed figure 

 (Fig. 1) ; thin (foreign) post-cards do very well indeed. Fold it 

 along the lines a a' and b b', then along c c' and d d', taking care 

 to fold always the same way. Then make the folds A A', BB', 

 C C, D D', still folding the same way. To do this you apply 

 A c against A a, and pinch out the line A A', and so on for the 

 remaining angles. This done, you have an imperfect tray with 

 " dogs' ears " at the angles. To finish it, turn the dogs' ears round 

 against the ends of the box, turn down outside the projecting 



