APPENDIX. 393 



support of the mass. This constitutes interstitial embedding, as distinguished 

 from superficial, by which the object is merely surrounded with some ma- 

 terial, as tallow and wax, that affords superficial support but does not bind 

 together the structural components. In either case, whether celloidin or par- 

 affin, the tissue must be carefully dehydrated before being placed in the fluid 

 embedding material. Upon the proper carrying out of this step depends the 

 success or failure of the embedding, since if water still remains within the ob- 

 ject, the embedding substance fails to penetrate its deeper parts and the 

 interstitial embedding is incomplete. 



Dehydration must be thorough, for unless all the water be extracted 

 from the object the subsequent penetration of the embedding substance will 

 be imperfect and the sections uneven or torn. This essential step is effected 

 by alcohol. Assuming that the tissue has been stored in 70 or 80 spirit, the 

 usual strength for this purpose, it is transferred to a tightly-stoppered wide- 

 mouth bottle, of 200 cc. capacity, containing 95 alcohol, for 24 hours. It 

 is then placed in a similar bottle, of 100 cc. capacity, containing absolute 

 alcohol, for 2448 hours, during which the absolute alcohol should be 

 changed. The exact length of time needed for complete dehydration varies 

 with the density and the size of the object, delicate small objects, such as 

 small embryos, evidently requiring much less time than compact and large 

 ones. It is always better to allow ample time for the extraction of the water, 

 since undue haste in this particular may require tedious retracing of subse- 

 quent steps. 



Celloidin Embedding. As supplied by some makers, celloidin is 

 purchased in small hard pieces, or " shreds," which are packed, surrounded 

 by water, in ounce lots in wide-mouth bottles. Although immersed in water, 

 the celloidin does not absorb the fluid and, after drying its surface, is really 

 free from moisture and desiccated. In order to remove the adherent water, 

 the shreds are allowed to dry in the air, protected from dust, or they are 

 first washed with alcohol to hasten the process. When perfectly dry, the 

 shreds of celloidin are broken into small pieces and dissolved and solutions 

 prepared as follows: 



Solution A. Dried celloidin 16 grams, dissolved in mixture of absolute 

 alcohol and ether, 100 cc. each. When completely dissolved, which re- 

 quires some days with occasional stirring, the solution has the consistence of 

 thick syrup; it is necessary, therefore, to prepare a less concentrated one. 

 Solution B is made by taking half of solution A and diluting with 50 cc. 

 each of absolute alcohol and ether. Solution C, made in a similar way by 

 taking half of B and diluting with 25 cc. each of absolute alcohol and ether, 

 is useful for dense tissue, as it insures thorough penetration. It is, however, 

 often omitted, the object being placed at once into B. 



The tissue having been thoroughly dehydrated, the actual embedding is 

 accomplished by the following manipulations: 



1. Dehydrated tissue placed in mixture of equal parts of absolute alco- 

 hol and ether for 24 hours. 



2. Transfer to thin (C) celloidin solution for 24 hours. 



3. Transfer to medium (B) celloidin solution for 24-48 hours or longer. 



4. Transfer to thick (A) celloidin solution for at least 24 hours ; may 

 remain in this for much longer time, often with advantage for a week if the 

 object be large. 



5. Mount on small blocks of vulcanized fibre, one surface of which is 

 first coated with a thin layer of thick celloidin solution. After arranging the 

 object in position, making sure that it rests firmly on the block by a broad 



