METHODS OF EMBEDDING. 411 



brass or copper, resting on a tripod and heated at one end by a 

 burner. When the latter method is employed the paraffin is melted 

 in a little glass dish, which is moved along the plate until a point 

 is found at which the paraffin remains melted at the bottom, but is 

 covered at the edges of the surface with a thin layer of congealed 

 paraffin. 



The length of time that the specimens should remain in the 

 melted paraffin will vary with the character of the tissues and the 

 method of getting rid of the alcohol which has been employed. It 

 should not be protracted longer than necessary for complete impreg- 

 nation, as heat is injurious to the tissues. When xylol has been 

 used two hours will usually suffice if the pieces of tissue are small, 

 and especially if they are transferred to a fresh paraffin-bath after 

 about an hour. This renewal of the paraffin is still more important 

 if oil of cedar-wood has been used. Chloroform requires a little 

 more time than xylol, and should be transferred to fresh paraffin 

 once or twice. 



When impregnation has taken place and the final bath of paraffin 

 no longer has the slightest odor of the clearing-agent the pieces of 

 tissue are removed from the bath with warmed forceps and placed 

 on bits of writing-paper, to which they adhere. A designation 

 of the specimen may be written on these papers, and the tissues 

 kept in this condition until required for cutting. They must then 

 be embedded. 



Methods of Embedding. 



The object of embedding is to surround the piece of tissue from 

 which sections are to be cut with a mass of the embedding-sub- 

 stance, which then not only supports the tissue when it comes in 

 contact with the knife, but also affixes it to a block or other support 

 which can be fitted into the clamp of the microtome. 



Microtomes designed for cutting paraffin usually have special 

 supports for the embedded specimen, but blocks of hard wood may 

 be used in their place. 



For the support of tissues embedded in collodion blocks of plate- 

 glass are probably both better and cheaper than those made of other 

 materials. They may be easily prepared from waste pieces of plate- 

 glass, about a quarter of an inch thick, and " obscured " or ground on 

 one surface. The glass may be cut into blocks of any desired size by 

 scoring the smooth side with a diamond and then splitting the pieces 

 apart with a sharp blow from a wedge-shaped hammer. The em- 



