76 CHAPTER yri. 



In any of these processes the material used for imbedding 

 is technically termed an " imbedding mass." 



There ai-e two chief methods of imbedding — the paraffin 

 method and the celloidin or collodion method. 



The pai-affin method is the one in most use ; for it is the 

 more rapid, requiring only hours where the celloidin process 

 requires days or weeks ; and it is the one which the most 

 readily affords very thin sections. Bat 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 .^plit under the impact of the knife. This 

 defect is, howevei-, much reduced by the employment of a 

 softer paraffin than is usual. In this way Strasser {Zelt. 

 ivifts. Mile, ix, 1892, p. 7) has obtained series of frontal 

 sections 30 /i thick through the entire human brain, in 

 paraffin blocks measuring 10 x 15 cm. And Mayee, with 

 the 'i'etrander microtome, has obtained series of only 7'5 (U 

 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 fransparent mass (which 

 facilitates orientation of the object), and of producing less 

 slirinlcage than paraffin (paraffin unavoidably shrinks on 

 cooling to at least 12 per cent.). It is for these two reasons 

 that celloidin is so frequentlj'^ 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 fixecl in jiosition in it whilst empty by 



