148 HISTOLOGY OF THE TEETH. 



ning of the development of the teeth, and the last being as 

 practicable for study in the pig. 



To Prepare the Material.— Disarticulate the inferior maxilla, 

 and place each in a separate bottle filled with a i per cent, 

 solution of chromic acid. Suspend the jaw in the fluid by means 

 of a string ; label, cork, and put away. After the jaws are com- 

 pletely decalcified — which can be ascertained by trying to pierce 

 them with a needle — they may be imbedded in one of three ways. 



First, placed between pieces of hog's liver, previously hardened 

 in alcohol, and thin sections cut with a razor ground flat on one 

 side. Very good sections can thus be made. 



Second, make a thin mucilage by dissolving picked 

 gum-acacia in warm water. Place the tissues in water for 

 twelve hours, then put them in the mucilage overnight ; 

 remove and place in alcohol to precipitate the gum ; hold 

 between pieces of liver and carrot, and make sections. 

 This method keeps the parts fairly well, which is of great advan- 

 tage in the study of the development of the teeth. The 

 disadvantage is that the sections must be placed in water to 

 remove the gum before staining, with the consequent risk of dis- 

 placing the parts. They can be mounted in glycerine, and studied 

 without any other staining than that which they have received 

 from the chromic acid. But I cannot recommend them, as they 

 do not make good, permanent specimens. 



Third, a new method of imbedding, introduced by Scheifer- 

 decker, viz. — celloidin. Celloidin is a perfectly pure preparation 

 of pyroxyline. I think the best method of preparing this mass is 

 to make a saturated solution of celloidin, with equal parts of 

 ether and alcohol. The decalcified specimen is first placed in 

 equal parts of ether and alcohol for twenty-four hours, and then 

 soaked for the satne time in the celloidin, so as to thoroughly 

 penetrate the tissue with the celloidin. Next, make a small paper 

 box, in which place the specimen and cover with a thick celloidin 

 solution ; place under a bell-glass, raised slightly from the table, 

 and leave overnight. The imbedding mass will shrink very 

 considerably, for which allowance must be made in the size of the 

 box and the cjuantity of mixture used. The shrinkage is due to 



