— 142— 



as the\' are thoroughly permeated with tlie celloidin; no 

 definite time can be fixed for this. Loose tissues, such as 

 lung, are permeated in 3 or 4 days; skin in 2 or 3 weeks; 

 brain or spinal cord, especially if the pieces are large, in 3 

 to 6 weeks. 



As soon as permeation seems complete, some of the 

 stock solution is poured into a flat glass dish (enough 

 to cover well the tissues to he em])edded), into this the tis- 

 sues are brought, and if several pieces are to Ije embedded, 

 they are to be so arranged that a small area of celloidin 

 (one-fourth of an inch wide) surrounds each piece. The 

 dish is now covered with a glass plate, and placed under a 

 bell-jar. The alcohol and ether evaporate very slowly, as 

 this goes on the celloidin hardens. In case the glass plate 

 fits tightly, push it to one side, thus leaving a small space 

 uncovered. Several days are required for the celloidin to 

 become hard. As soon as the mass is so firm that it can 

 not be indented with the finger, it is removed from the 

 glass dish. It can now be cut into a square block contain- 

 ing the tissue; or into several, if a number of pieces were 

 embedded. These celloidin blocks are to be fixed to small 

 cylinders of hard wood. The diameter of the wooden 

 blocks should be a little larger than the celloidin block, 

 and about one-half an inch long. This is done by immers- 

 ing one end of the wooden cylinder in the stock solution; 

 a layer of celloidin is in this way spread over one end of 

 the cylinder of wood. The celloidin block is, after having 

 been immersed in equal parts of alcohol and ether for about 

 5 minutes, pressed against the layer of celloidin on the 

 wooden blo'ck, and then placed under a bell-jar for about 

 an hour. It is then transferred to 70% alcohol for several 

 hours, at the end of which time it is ready for cutting. 



When cutting sections of a tissue embedded in celloidin, 

 the knife should be moistened with 80% alcohol; sections 

 are cut into weak alcohol or distilled water. The celloidin 

 blocks, even after they are fixed to the wooden cylinders, 

 may be kept for a long time in 70% alcohol. 



The celloidin method is now ver}^ largel}^ used ; it is 

 especially useful for cutting sections of the central nerv- 



