APPENDIX. 



395 



ure of the two, so proportioned that the resulting mass has a melting point 

 of about 52 C. The proportions vary so with the season and temperature 

 of the laboratory that no exact figures can be given, but, in a general way, 

 about equal parts will yield a satisfactory mass for ordinary use. When the 

 object is relatively hard and very thin sections are desired, harder paraffin is 

 employed than usual. A nice adjustment of the consistence of the embed- 

 ding mass to the character of the tissue and the particular purpose in view is 

 an important factor in securing satisfactory results. 



The manipulations in paraffin embedding are as follows : 



1. The thoroughly dehydrated tissue is transferred from absolute 

 alcohol to: 



2. Chloroform, 4-24 hours, depending upon the size of the object, 

 small delicate embryos requiring often only 1-2 hours. When the alcohol is 

 completely replaced, the tissue floats below the surface or sinks. 



3. Saturated solution of paraffin, in chloroform, 6-24 hours, depend- 

 ing upon the size and density of the object. 



4. Melted paraffin in small open porcelain or glass round-bottom 

 dish, 2-12 hours, in oven at 52-54 C. The paraffin must be carefully 

 guarded against overheating to prevent injury to the tissue. After 1-2 

 hours, the object should be transferred to a second capsule with fresh melted 

 paraffin. So long as chloroform is present, the embedding is incomplete. 

 In order to test these conditions, a rod may be judiciously heated and held 

 for a few moments in the paraffin in the vicinity of the object, when, if still 

 present, the chloroform is liberated as small bubbles. When the chloroform 

 has been driven off, the object is transferred once more to fresh paraffin pre- 

 paratory to embedding. This last transference is delayed until there is reason 

 to believe that the object is completely saturated with the embedding mass 

 and free from chloroform, since the presence of the latter is unfavorable to 

 the desired homogeneity of the embedding mass. 



5. Embedding is accomplished by surrounding the object with melted 

 paraffin in a folded paper box or, still better, an adjustable metal frame. 

 When an embedding frame is employed and its convenience recommends 

 it it is adjusted to suitable size and placed upon a piece of glass resting on 

 the flat bottom of a deep dish. The frame and sheet of glass should be 

 warmed to prevent the too rapid solidifying of the paraffin. The object 

 may be transferred to the embedding box by pouring when the box is 

 filled, or by transferring with a warmed loop of platinum wire. Before 

 the paraffin ceases to be fluid, the position of the object, with regard to 

 the desired planes of section, must be carefully adjusted by means of heated 

 needles. 



When the proper orientation of the object has been secured, cold 

 water is poured into the dish, care being taken neither to shake the object, 

 and thereby disturb its position, nor to allow the water to rise above the 

 sides of the frame. The mass is allowed to remain for a few moments, 

 until the surface of the paraffin is completely covered by a thin pellicle of 

 congealed substance. When this has occurred, the water is very gently 

 added and the entire frame and contents submerged. If the water be 

 allowed to come into contact with the still fluid paraffin, cavities containing 

 water may be imprisoned within the block an accident that may interfere 

 with proper cutting. In order to secure homogeneous paraffin, a most 

 desirable feature for satisfactory sectioning, it is necessary to solidify the 

 melted mass as rapidly as may be done with prudence. If it be allowed to 

 cool slowly, crystallization occurs and the mass becomes opaque and friable. 



