FIXING AND MICKOTOME SECTIONING 243 



into pietH's as small as cau be convenieutly handled. These may 

 theu be prepared by the following processes: 



1. Fixing: If the material is to be used simply for the study of 

 tissue-arrangement, cell-structure, etc., the treatment with alcohol 

 described in the paragraph relating to the preparing and keeping of 

 laboratory material is sufficient preparation for the imbedding process. 

 Protoplasmic structures, however, are likely to be distorted or disin- 

 tegrated after this treatment, due to the slow process of killing. Some 

 method of quickly killing or" fixing" the protoplasm is therefore neces- 

 sary. With hematoxylin staining only a few methods are available, 

 among which the following is perhaps the best. Cut the fresh material 

 into very small pieces (the smaller tlie better) and drop into so-called 

 absolute alcohol (96 per cent or stronger) ; after a few hours preserve 

 in 90 or 95 percent alcohol. With otiier stains more accurate fixing 

 agents may be used, such as chromic acid, osmic acid, acetic acid, 

 etc., either separately or in combination. The treatment, however, 

 is in these cases rather complicated. 



2. Imbedding: The pieces must be imbedded in some substance 

 in which tliey cau be held and sectioned. For this, collodion is 

 used for harder tissues. Pour off the alcoliol, and add enough 2 

 per cent collodion to cover the material about three -fourths of an 

 inch. After twenty-four hours tiiis may be poured back into the 

 stock bottle, and an equal amount of 5 per cent collodion put on the 

 material. The collodion contains ether and alcohol, both of which 

 are volatile; therefore these operations must be pei'formed as quickly 

 as possible, and the corks of collodion bottles should always be 

 sealed by holding the bottle neck down for a few seconds. Leave 

 the material in 5 per cent collodion twenty-four hours, and then 

 pour the contents of the vial into a paper box, which may be made 

 by folding a piece of writing paper. The size of the box must be 

 judged so that each piece of material will be surrounded by a 

 quantity of collodion, and the inside of thd box should be greased 

 with vaseline to prevent the collodion from sticking. The pieces 

 will sink to the bottom, where they may be arranged with a needle. 

 If there is not enough collodion in the box add some from the stock 

 bottle. The box should then be placed in a shallow vessel on the 

 bottom of which a little alcohol has been poured, and covered with 

 a pane of glass leaving a very small opening on one side. In about 

 twenty-four hours the collodion will have hardened into a cake hav- 

 ing the consistency of cheese. The material may now be cut into 

 small blocks and stored in 85 per cent alcohol. 



3. Cutting: For cutting sections, either a hand microtome or a 



