95 

 IMBEDDING METHODS. 



§ 40. When the consideration of time is not so important and 

 finer results are more to be desired, the sections should be prepared 

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

 The interstices of the tissue are completely filled with some sub- 

 stance that will give support and greater consistency and homegen- 

 eity 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 Collodion method, and (c) the Freezing 

 method ; the imbedding masses to fill the spaces being respectively 

 paraffin, collodion and a congelation mass, — ice. The last is the 

 simplest ; it requires less expenditure of time, fewer reagents, and 

 its results are the crudest. As in cutting free-hand sections without 

 imbedding, the freezing microtome should be employed 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 determined by the 

 following considerations and the special exigencies of the case : 



(i) As a general rule paraffin may be employed when very 

 thin sections are desired and the piece or organ is not very large ; 

 collodion, when larger sections are necessary but which need not be 

 so thin. (2) For paraffin, heat is necessary, which is not required 

 in the collodion method. (3) In the case of paraffin, the imbedding 

 mass is removed from the sections before they are stained and 

 mounted ; the collodion is (usually) not dissolved out. With most 

 nuclear stains the collodion is colored more or less, affecting the ap- 

 pearance and excellence of the preparation unless it is bleached, a 

 process not possible in many cases. (4) In paraffin sections there 

 is apt to be more or less crushing together of the parts of the tissue ; 

 by the employment of collodion, the/orw of the organ may be more 

 exactly preserved, sometimes an important consideration. 



§ 41. The following will indicate the steps in the employment 

 of the two methods : 



