196 COLLODION AND OTHER IMBEDDING METHODS. 



300. Imbedding. At this stage, if it has not been done 

 "before, the objects must be imbedded that is, arranged in 

 position in the receptacle in which they are to be hardened. 

 For the usual manipulations see 273. I recommend the 

 paper thimbles or cylindrical trays, Fig. 2, as being very 

 convenient for collodion imbedding. The bottoms, however, 

 should be made of soft wood in preference to cork ; cork is 

 elastic, and bends in the object-holder of the microtome, de- 

 forming the mass and object. The box should be prepared 

 for the reception of the object by pouring into it a drop of 

 collodion, which is allowed to dry. The object of this is to 

 prevent bubbles coming up through the wood or cork and 

 lodging in the mass. 



Objects may also be imbedded on a piece of pith or leather, 

 which should also be prepared with a layer of dry collodion. 



Watch-glasses, square porcelain water-colour moulds, and 

 the like, also make convenient imbedding receptacles. Care 

 should be taken to have them perfectly dry. Any of these 

 receptacles or supports may be set with the mass under a 

 glass shade, allowing of just enough communication with the 

 air to set up a slow evaporation. Or porcelain moulds or 

 small dishes may be covered with a lightly fitting cover. 



If it be desired to mark the position of the object in the mass in order to 

 facilitate the subsequent orientation of it on the object- holder of the micro- 

 tome, recourse may be had to the method described by ETCLESHEIMEE in 

 Amer. Nat., xxvi, 1892, p. 354 (see also Journ. Boy. Mic. Soc., 1892, 

 p. 563). The object is imbedded in one of the metal boxes described in 

 273. The box has its ends and sides perforated at regular intervals by 

 small opposite holes. Silk threads are passed through these holes from side 

 to side, stretched, and kept tight by sticking them to the sides of the box by 

 means of a drop of celloidin, leaving a length of a couple of inches hanging 

 loose. The loose ends are soaked in thin celloidin solution with which lamp- 

 black has been mixed. The object is arranged in position on the framework 

 formed by the taut threads in the box, the mass is poured in, and the whole 

 is hardened. After hardening, the celloidin holding the ends of the threads 

 is dissolved by means of a drop of ether, and the lampblacked ends are pulled 

 through the box. This leaves adhering to the bottom of the mass a series 

 of black lines which form useful orientation points. 



301. Hardening. This is logically the next step, but as a 

 matter of fact is frequently begun before. For the different 

 processes of the collodion method so run into one another tbat 

 it is difficult to assign natural lines of demarcation between 

 them. 



