Ch. XI] PREPARATIONS BY THE PARAFFIN METHOD 375 



soon freeze the mucilage and the tissue, as shown by the white appear- 

 ance. When frozen, cut the tissue rapidly. It is well to have an 

 assistant turn the feed screw up while the sections are cut. When 

 20 or 30 sections are cut place them in water or normal salt solution. 

 This is a rapid method of getting sections much used in pathology 

 where quick diagnoses are demanded. In normal histology the freez- 

 ing microtome is used mostly for organs or parts of greatly varying 

 density. For example, if one wishes sections of the finger and finger 

 nail, this apparatus offers about the only means of getting good 

 sections. In that case the bone is decalcified before trying to make 

 the sections (§ 559). 



Frozen sections are also very useful for demonstrating the presence 

 of fat by staining with Sudan III. 



The Paraffin Metijod of Sectioning 

 § 610. Object of the paraffin. — In the early periods in histology 

 great diflBiculty was encountered in making good sections of organs 

 and parts of organs, because the different tissues were very unlike in 

 density. At first tallow and beeswax, elder pith, liver, and various 

 other substances were used to enclose or surround the object to be 

 cut. This gave support on all sides, but did not render the object 

 homogeneous. In the early sectioning, a great effort was made to 

 keep aU imbedding material from becoming entangled in the meshes 

 of the tissue. This was guarded against by coating the object with 

 mucUage, and hardening it in alcohol. This mucilage jacket kept 

 the tissue free from infiltration by the imbedding mass ahd itseK was 

 easily gotten rid of by soaking the sections in water. 



A great advance was made when it was found that the imbedding 

 mass could be made to fill all the spaces between the tissue elements 

 and surround every part, the tissue assuming a nearly homogeneous 

 consistency, and cutting almost like the clear imbedding mass. Cocoa 

 butter was one of the first substances to be used for thus "infiltrat- 

 ing" the tissues. The imbedding mass must usually be removed 

 before the staining and mounting processes; but in staining for 

 glycogen by the iodin method, the stain is applied before the paraf- 

 fin is removed (§575). 



