BY DR. KLEIN. 105 



dried before embedding, otherwise the mass will not adhere 

 to it. 



As regards the other fatty masses, the only one which can 

 be recommended is a mixture of five parts paraffin, two parts 

 spermaceti, and one of lard. It is, however, decidedly inferior 

 to the mass of wax and oil. 



Embedding in Gum or Gelatin. It has already been 

 stated that objects with delicate projections in close proximity 

 to each other (e. </., papillae or villi), can be better embedded 

 in gum or gelatin than in wax and oil. The wax-mass, in 

 solidifying, does not penetrate between the projecting parts, 

 so that they are unsupported, and consequently are apt to be 

 broken off in making sections. Gum is solidified by immersion 

 in alcohol, gelatin by cooling : in both cases the process is so 

 slow that the mass has time to penetrate between the inequali- 

 t ties of the surface of the object. The gum or gelatin solution 

 must be concentrated ; to the gelatin a little glycerin should 

 be added. I think gum preferable, first, because the consist- 

 ence of the solid mass can be varied according to the time it 

 is left in alcohol ; and even if it has already become too hard, 

 it may be softened by adding to the alcohol a few drops of 

 water. No such modification is possible in the case of gelatin. 

 It is also more easy to make sections in gum than in gelatin, 

 the elasticity of which is a great disadvantage. On the other 

 hand, it is easier to embed in gelatin, and the time required for 

 solidification is much shorter. The method of embedding in 

 gum is as follows : A thick solution of powdered and sifted 

 gum arabic is prepared in a beaker, and allowed to stand in a 

 water-bath until all air-bubbles have collected at the surface in 

 the scum, which must then be removed by skimming ; after 

 which the solution may be used. A little box of paper is then 

 prepared, of suitable size, which is placed on a plate of cork. 

 The bit to be embedded is then stuck through with a needle, 

 the point of which is thrust into the cork through the bottom 

 of the box ; the same rules being followed as regards the posi- 

 tion of the bit in the box as in embedding in wax-mass. The 

 whole is then transferred to a glass capsule. As soon as the 

 bit is nearly dry at the surface, the solution is poured along a 

 glass rod into the box until it is full to the brim. Alcohol is 

 then carefully poured into the capsule, until the little box is 

 immersed to half its height. The whole must then be covered 

 over and left for two or more hours. As soon as the gum be- 

 comes opaque and white on the surface, which occurs in about 

 the time mentioned, the whole mass can* be immersed in alco- 

 hol until it is brought to the required degree of solidity. The 

 process may be accelerated either by changing the alcohol 

 frequently, or by using absolute alcohol. If the mass is too 

 hard, it can be softened by adding a drop or two of water to 



