412 HISTOLOGICAL TECHNIQUE. 



bedded specimen is affixed to the rough surface of these blocks by 

 means of collodion, and the blocks may be numbered with a lead 

 pencil upon the rough surface. The writing will be preserved from 

 obliteration by the specimen subsequently placed upon it, and can 

 be read through the glass. 



1. Embedding in Collodion (or Celloidin). Tissues of firm con- 

 sistency and moderately uniform structure, such as liver, kid- 

 ney, and the majority of tumors which have been hardened, 

 may be embedded without previous impregnation. Before this 

 can be done, however, they must be either dehydrated with abso- 

 lute alcohol, or soaked for a few hours in a mixture of equal 

 volumes of ether and alcohol (95 per cent, alcohol will answer, 

 if absolute alcohol is not to be had). For this rapid method the 

 bottom of the piece of tissue must be flat and parallel to the plane of 

 the desired sections. When the necessary trimming of the speci- 

 men is completed moisten it with absolute alcohol or the ether- 

 alcohol mixture, then dip it in the thick solution of gun-cotton and 

 place it at once upon the ground surface of the glass block (pre- 

 viously labelled). In a few minutes the collodion will have evap- 

 orated sufficiently for the formation of a distinct pellicle upon its 

 surface. When this has become firm enough to withstand gentle 

 pressure immerse the block and specimen in several times their vol- 

 ume of 80 per cent, alcohol. This will harden the collodion, and in 

 the course of a few hours the specimen will be ready for cutting. 



Tissues impregnated with collodion had best be embedded by a 

 slower process than the foregoing, although that method will answer 

 where only a slight support of the tissue-elements within the speci- 

 men is needed. A gradual concentration of the collodion within 

 the tissues may be brought about in the following manner : 



Smear the inside of a small, straight-sided glass dish with a trace 

 of glycerin and then fill it with enough moderately thick collodion 

 to cover the pieces of tissue with a layer about one-quarter of an 

 inch deep. Now place the specimens that have been in thin collo- 

 dion in the dish, with the surfaces from which sections are to be cut 

 resting on the bottom. Place the dish in a larger vessel with higher 

 sides and loosely cover the latter. The ether and alcohol in the 

 collodion will gradually evaporate, and their vapors will first fill the 

 outer vessel and then overflow its sides. The depth of the outer 

 vessel keeps these vapors in contact with the surface of the collo- 

 dion, preventing the formation of a pellicle, which would retard 



