MUSEUM OF COMPARATIVE ZOOLOGY. 47 



paraffin has cooled, the metal guards are carefully removed, and the 

 paraffin is cut away until the edges of the paper are exposed. This is 

 an important preparation for the next step. The glass slip with the 

 attached block of paraffin is then put into a vessel of water. The water 

 working into the paper from its edges dissolves the gum-arabic, and the 

 block is thus detached from the glass. The film of gum between the 

 collodion and the paper is also dissolved, so that the paper can then be 

 removed, leaving the block of paraffin with the imbedded object. The 

 object thus lies close to the lower surface of the block, over wbich is the 

 delicate film of collodion and the imprint of the grain of the paper. One 

 face of the paraffin block, which is a plane surface, thus bears a cast of the 

 grain of the paper, the lines of which have definite relations to the axes 

 of the imbedded object. It is now a simple matter to orient the block 

 in the microtome so that the lines on the face of the block will have any 

 position in relation to the plane of section. A whole series of objects 

 can thus be oriented on one strip of paper ; and with a fine pen numbers 

 referring to notes and drawings can be marked on the collodion surface 

 adjacent to each object. These marks will appear on the face of the 

 paraffin block after imbedding. The film of collodion on the face of the 

 block in no way interferes with making good '" ribbons," for, owing to 

 the fact that the collodion was much diluted, the film is so fine that it 

 offers practically no resistance to the knife in cutting, and does not 

 endanger the specimen. 



Although necessitating a rather long description, the method is ex- 

 tremely simple. The chief advantages are: (1) the easy orientation 

 of objects when filled with ^ome clearing medium, by which details 

 are made more visible ; (2) the conspicuousness of the objects through 

 all stages of the process ; (3) the saving of time in being able to imbed 

 many objects at one operation ; and (4) the absence of any necessity for 

 haste. The method has been tested by others with various objects, and 

 always with success. 



Museum of Comparative Zoology, 

 May 10, 1893. 



