72 IMBEDDING METHODS. 



sections. But this only applies to fairly small objects : with objects 

 of much over, half an inch in diameter you cannot 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 ju, 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 // with a surface 

 of 4J x 3 cm. 



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

 and presents advantages for the manipulation of the sections ob- 

 tained. 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. 



137. 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 f , then along c c and d d f , taking care to fold always 

 the same way. Then make the folds A A', B B',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 



