294 NORMAL HISTOLOGY. 



For a room-temperature of 20° C. (68° Fah.) a variety melting at 

 50° C. (122° Fah.) will give good results. If the laboratory is 

 warmer, a paraffin of* higher melting-point should be used. 



Impregnation is accomplished by placing the bits of tissue in a 

 bath of melted paraffin maintained at a temperature only slightly 

 above that of fusion, say 52° C. (125.6° Fah.), if the paraffin melts 

 at 50° C. (122° Fah.). This may be accomplished in a water- 

 jacketed oven provided with a thermoregulatory or upon a plate of 

 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, especially if the pieces of tissue 

 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 about the same time as xylol, 

 and the tissues 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 are ready 

 for immediate embedding (vide infra), or may be placed on smooth 

 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 then are embedded. 



Methods of Embedding 1 . 



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 



