296 NORMAL HISTOLOGY. 



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 

 evaporation and also favor the formation of bubbles in the collo- 

 dion. After an interval of one or more days the collodion will have 

 a gelatinous consistency. It should be allowed to become so hard 

 that it has considerable firmness, but is still soft enough to receive 

 an impression of the ridges in the skin when pressed with the 

 finger. The outer vessel is then partly filled with 80 per cent, 

 alcohol so that the whole of the inner dish is submerged. 



By the next day the collodion will be hard enough for removal 

 from the dish. With a small scalpel, held vertically, divide the 

 hardened mass of collodion into portions, each of which contains 

 one of the pieces of tissue (for several pieces may be embedded in 

 the same dish, provided care be taken to preserve their identity). 

 Remove the pieces and trim down the collodion around the speci- 

 mens, leaving a margin of about an eighth of an inch. Trim the 

 top surfaces of the collodion parallel with the bottom surfaces, then 

 dip the trimmed surface into a little absolute alcohol contained in 

 a watch-glass, in order to dehydrate it. This will take about two 

 minutes. Label glass blocks with lead-pencil, place a drop of 

 thick collodion on the writing, and transfer the embedded specimens 

 immediately from the absolute alcohol to the drop of collodion, 

 pressing it into contact with the glass. When a good pellicle has 

 formed on the collodion drop the whole block into 80 per cent, 

 alcohol. If the block of hardened collodion containing the speci- 

 men be sufficiently dehydrated on the surfaces coming in contact 

 with the drop of collodion, and the latter have not time for the 

 formation of a pellicle before it receives the block, there will be no 

 difficulty in cementing the embedded specimen to the roughened 

 surface of the glass. It is best not to cut sections until the day 

 after the specimen has been affixed to the glass block. These 

 blocks, with attached specimens, may be preserved indefinitely in 

 80 per cent, alcohol. 



