20 



according to some method in which an imbedding mass is used. The 

 interstices of the tissue are completely filled with some substance 

 that w r ill give support and greater consistency and homogeneity to 

 the tissue, and thereby enable the cutting of much thinner and more 

 perfect sections. 



There are three methods that are generally employed, (a) the 

 Paraffin method, (b) the Celloidin (Collodion) method, and (c) the 

 Freezing method; the imbedding masses to fill the spaces being re- 

 spectively paraffin, collodion and a congelation mass, ice. The last 

 is the simplest; it requires less expenditure of time, fewer reagents, 

 and its results are in some ways the crudest. As in cutting free-hand 

 sections without imbedding, the freezing microtome should be em- 

 ployed when haste is necessary and finer detail unimportant, as in 

 clinical work. The two remaining methods may be employed in most 

 cases and give good results. A choice between them mUst be deter- 

 mined by the special requirements of the case and a consideration of 

 the differences of the two methods as set forth in tabular form below : 



CELLOIDIN. PARAFFIN. 



No heat required. Heat required. 



Sections relatively thick ; 10 to 25 mi- Sections relatively thin ; 1 to 10 microns 



crons or more. or more. 



Imbedding mass usually not removed. Imbedding mass removed. 



Sections usually cut wet (with alcohol or Sections cut dry. 



oil). 



Knife oblique. Knife usually set at right angles. 



Cutting stroke slow. Cutting stroke usually rapid. 



Form of the organ better preserved. Usually more or less distortion. 



Imbedding requires more time. Imbedding requires less time. 



Celloidin stains with basic dyes. 



In general, better for larger specimens. Better for smaller objects. 



48. Despite the differences, the two methods may in most 

 cases be used interchangeably. The advantages of the paraffin 

 method are (1) the facility of its use and the ease with which thin 

 sections may be obtained and its adaptation to serial sectioning. 

 Celloidin is particularly useful when heal is injurious or the paraffin 

 solvents (clearers) dissolve out substances which it is desired to 

 preserve. Its main defect is the readiness with which the celloidin 

 stains with basic stains, particularly such as Iron Hematoxy lin and 

 the coal-tar dyes (safranin, gentian violet, methylene blue, etc.) 

 The celloidin may, however, be dissolved out. See 136. 



49. The following table will indicate the steps in the employ- 

 ment of the two methods: 



