CELLOIDIN 103 



not to loosen the cork while the celloidin is above room temperature, 

 otherwise the tissue will be ruined by the rapid formation of ether- 

 alcohol bubbles released when the pressure is suddenly reduced. 

 The rapid infiltration of this newer method which has been 

 adapted from the more elaborate botanical techniques, see 

 Wetmore {Stain Tech., vii, 1932, p. 37), is a great advantage, but 

 it is possible, in a few special cases, such as the Organ of Corti 

 in adult mammals, that heating may cause distortion of delicate 

 structures which is not found after infiltration at room temperature. 



187. Imbedding. The aim in imbedding is to cast the tissue 

 in a solid block of celloidin which is concentrated to a degree so 

 that it no longer responds to squeezing or retains a finger-print 

 impression. It must be emphasised immediately that successful 

 sectioning depends very largely on the final consistency of the 

 block. Thin sections are more easily cut from a very rigid block. 

 Very tough tissues require a very dense imbedding medium, 

 whereas softer tissue, like brain and material to be cut in thick 

 sections, may be imbedded in a somewhat softer block. 



188. Concentration by Evaporation. Imbedding is most con- 

 veniently carried out in a paper boat, coated with thin celloidin 

 which is allowed to dry, and then filled with 8 to 16 per cent, 

 celloidin. The tissue is pushed down into the celloidin and 

 orientated roughly (finer orientation is best done by subsequent 

 trimming of the translucent block). If bubbles develop in the 

 celloidin, the paper boat can be transferred for a short period to a 

 vessel containing ether vapour, Busse {Zeit. iviss. Mikr., viii, 1892, 

 p. 467). The bubbles quickly rise to the surface. The boat is then 

 transferred to an empty glass receptacle with the lid adjusted so 

 that very slow evaporation and concentration of the celloidin 

 may proceed. The process appears to involve simply a precipi- 

 tation of the celloidin owing to the loss of ether from the solvent. 

 Lee recommends that the evaporation should take place under a 

 bell- jar in an atmosphere of alcohol vapour. This is especially 

 indicated for large blocks. After a time the celloidin shrinks down 

 and more thick solution must be added if the surface of the tissue 

 tends to become exposed. When the celloidin is sufficiently 

 hardened the paper can be stripped off to allow even evaporation 

 on all sides of the block. If the " setting " is slow, remove the 

 lid from the container and displace some of the vapour by blowing 

 air into the vessel and then replace the lid. The whole process of 

 adding fresh celloidin and placing the objects under the required 

 conditions of evaporation is repeated every few^ hours and may be 

 continued for several days if necessary. 



Apathy {Zeit. wiss. Mikr., v, 1888, p. 47) arranges objects on a 

 small rectangular plate of gelatin, when orientation is difficult. 

 The gelatin plate is placed on the bottom of the imbedding vessel. 

 It is turned out with the celloidin mass after hardening and cut 



