156 Invertebrate Zoology. 



somewhat smaller in area than the block from which it is 

 cut. The sections are too thin, or the room is too warm, or, 

 quite likely, the knife has become smeared with dirt or par- 

 affin. Clean the knife on both sides by stroking it from the 

 back toward the edge with a clean cloth dampened with 

 chloroform. 



The Celloidin Method of Imbedding and Cutting. 



The object is first stained in toto, dehydrated, infiltrated with 

 thin, medium, and thick celloidin or collodion, and finally 

 placed in a paper tray filled with the thick collodion. In a 

 few minutes a film will form over the exposed surface of 

 the collodion, when the paper tray and its contents are 

 placed in a jar of strong chloroform, in which, after a few 

 hours, the collodion becomes quite hard. 



The tray is now taken from the chloroform and, after the 

 paper has been removed from the hardened block, the col- 

 lodion with its enclosed object is placed in a vial of white 

 oil of thyme or some similar oil. If the block of collodion 

 is not large, in a few hours it will become as clear as glass, 

 the stained object appearing as if suspended in a transparent 

 fluid. 



For the process of orienting, the block of collodion may 

 be taken from the oil, placed in a watch-crystal, and, after 

 covering with the oil of thyme, examined with a lens or, ii 

 very small, with a compound microscope. The side of the 

 block that is to be attached to the object-holder of the micro- 

 tome is now selected, wiped dry of the oil, and immersed 

 for a moment in ether, and then smeared with thick collo- 

 dion. The object-holder, a block of wood rather than cork, 

 is smeared in the same way, and the two collodioniz^d sur- 

 faces are brought together. 



