160 COLLODION AND OTHER IMBEDDING METHODS. 



is to penetrate them first with a thin solution, then with the 

 definitive thick one. 



If celloidin be taken the solutions are made in admixture of 

 ether and absolute alcohol in equal parts. If collodion be 

 taken the thin solution may be made by diluting it with ether. 

 Apathy recommends that celloidin be allowed to dry in the 

 air until it becomes yellow, transparent, and of a horny con- 

 sistency, and that it be then dissolved in the alcohol and 

 ether (sulphuric, free from acid). The solutions thus pre- 

 pared are free from the excess of water that is present in the 

 undried celloidin, and give after hardening a mass that is 

 more transparent and of a better consistency for cutting 

 (Zeit.f. wiss. Mik., vi, 2, 1889, p. 164). 



The objects ought to remain in the first bath until very 

 thoroughly penetrated; days, even for small objects, weeks, 

 or months, for large ones (human embryos of from six 

 to twelve weeks for instance). If the object contain cavities, 

 these should be opened to ensure their being filled by the mass. 



When the object is duly penetrated by the solution, it should 

 be brought into the thick one. SCHIEFFEKDECKER (I. c.) recom- 

 mends that this be done by allowing the thin solution to con- 

 centrate slowly (the stopper of the containing vessel being 

 raised, for instance, by means of a piece of paper placed under 

 it), and making up the loss from evaporation with thick 

 solution. 



293. 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 266. I recommend the 

 paper thimbles or cylindrical trays, fig. .2, as being very con- 

 venient 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 leuther, 

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



