41 8 APPENDIX. 



from 24 to 48 hours and then transferred to the " A" or thin celloidin 

 solution, in which it remains for several days, until entirely permeated 

 with the mass ; the tissue is then placed in the thick " B" solution, 

 where it stays until the thinner fluid has been replaced by the thicker. 

 Meanwhile, corks of suitable size should be soaking in the mixture 

 of equal parts of absolute alcohol and ether ; one of these is selected, 

 its end slightly roughened, and finally moistened with a few drops 

 of the mixture just before the tissue with an envelope of the thick 

 celloidin solution is placed in position for cutting on the cork, care 

 being taken that the stratum (1-2 mm.) of celloidin lies between the 

 tissue and the cork. After a few moments a fresh layer of celloidin 

 is added, and this process is repeated until the tissue is completely 

 surrounded with a stratum of the embedding mass ; or the tissue 

 may be completely embedded, after being attached to the surface of 

 the cork, by fastening a piece of writing-paper to the sides of the 

 cork and at once filling the resulting mould with the fluid celloidin. 

 The mass of fresh celloidin should remain undisturbed until the sur- 

 face has hardened sufficiently to prevent all possibility of shifting, 

 when the cork with the tissue is transferred to a vessel containing 

 75 per cent, spirit to harden, where it remains, completely immersed, 

 from i to 3 days or longer ; at the expiration of this time the block 

 has gained a consistence suitable for sectioning. The cutting can be 

 done either free-hand or by the microtome, but it must be done 

 while both knife and tissue are flooded with 70 per cent, alcohol. 

 The sections are transferred to 70 per cent, spirit for subsequent 

 treatment ; if already stained, they are passed into 95 per cent, for 

 dehydration, cleared in xylol, bergamot oil, or cedar oil (but not in 

 clove oil, as this dissolves the celloidin), and mounted in balsam ; if, 

 on the other hand, the tissue has not been stained in bulk, the sec- 

 tions are treated with the selected stain, alcoholic or aqueous, and 

 subsequently dehydrated, cleared, and mounted. 



5. Sectioning. While for the purposes of immediate examination 

 or of temporary preparation free-hand sections often suffice, yet no 

 one seriously undertaking histological investigation can afford to 

 ignore the advantages possessed by the approved microtomes of the 

 present day, without which accurate work is impossible. After an 

 extended experience with many forms of these instruments, the 

 writer unhesitatingly recommends the sliding microtome, as made by 

 Schanze, of Leipsic, as the best all-round instrument to be had, the 

 medium-sized " model B" of this maker supplying an ideal tool 

 capable of executing all forms of cutting in the most satisfactory and 

 convenient manner. The little sliding student's microtome made by 

 the Bausch and Lomb Optical Company answers as a very satisfactory 



