PREPA.RING SECTIONS OF TEETH. 31 



bulk of the immersed tissue. Change the fluid every day for four 

 or five days, and then every third or fourth day. Then finish 

 hardening after it has been in Muller's fluid for a fortnight, first in 

 weak spirit, then into strong, changing till no more colour is seen. 

 Vertical sections are cut by a thin, sharp knife ; place longitudin- 

 ally on the stage of a Cathcart or Williams' freezing microtome 

 and cut in the usual way. The best specimens are got in the 

 canine and bicuspid regions, for these parts are less likely to be 

 disturbed in the manipulation processes. Embedding in wax, 

 paraffin, or celloidin is of little service. The knife cuts the thin 

 cap of semi-calcified dentine and bone quite easily, and the ele- 

 ments of the pulp are in no way disturbed in their relation to each 

 other. The odontoblasts can be separated, if necessary, by sepa- 

 rating with the point of a needle from the surface of the dentine 

 papilla, the cap of dentine to which, in places, they adhere. This 

 affects little, if at all, the relative positions of dentine, odonto- 

 blasts, and pulp. 



To make Preparations of the Teeth of Fishes in situ.— It is 

 best not to grind down sections of the teeth, but to decalcify the 

 jaw and teeth with a 5 per cent, solution of hydrochromic acid or 

 a 10 per cent, solution of hydrochloric acid. Cut sections, stain 

 and wash them well in distilled water, dehydrate for three minutes 

 in absolute alcohol, clear in clove oil or xanthol, and mount in 

 Canada balsam. Carmine is, perhaps, the best stain for fishes' 

 teeth. If it is used, however, it is necessary, before transferring to 

 distilled water, to pass the sections quickly through weak acetic 

 acid, as this fixes the stain. If gold chloride is used, the speci- 

 mens must be mounted in glycerine jelly. 



It is not necessary to Cut Sections of Enamel to Demonstrate 

 the Prisms. — ist. Soften the enamel by immersion in a 10 percent, 

 solution of hydrochloric acid. By means of a needle point or 

 fine brush, remove a small portion to a slide ; put a drop of 

 normal salt solution on the top of the enamel and press down the 

 cover-glass ; then run a solution of carmine or orange-rubine 

 beneath the cover-glass, and draw off the excess with a small piece 

 of blotting-paper. Wash the stain away further by irrigation with 

 a weak hydrochloric acid or acetic acid solution, and mount in 

 this solution or acidified glycerine after Beale's plan. 



