14 NORMAL HISTOLOGY. 



interstices may be filled with some fluid material 

 which can afterward be rendered solid and resist- 

 ent, so that the whole forms a firm mass. For this 

 purpose celloidin is the most suitable. 



Cclloidin is a pure pyroxylin, and makes a clear 

 solution. It comes in the form of thin slabs or in 

 thin shavings ; the latter are to be preferred since 

 they dissolve more easily. For use, a saturated 

 solution of the celloidin shavings is made in a mix- 

 ture of equal volumes of alcohol and sulphuric 

 ether. This solution may be diluted to suit any 

 particular case. The specimen to be imbedded is 

 placed in alcohol and ether, from strong alcohol, and 

 allowed to remain in it for at least twelve hours. It 

 is then transferred to a dilute solution of celloidin, 

 where it remains for twenty-four hours. At the end 

 of this time it is placed in a saturated solution of 

 celloidin for one to seven days, according to the size 

 and density of the specimen. Loose tissues, like the 

 lung, require a less time than more dense tissue, like 

 the liver or kidney. When the specimen has become 

 thoroughly permeated, it is to be imbedded by one 

 of the following methods : 



a. Cover the smooth surface of a cork or a block 

 of wood with a moderately thick layer of celloidin, 

 and allow it to dry down hard. Then place the 

 specimen, which has been soaked in thick celloidin, 

 on this, and cover it, layer by layer, with a thick 

 solution of celloidin,, allowing each layer to partially 



