INFILTRATION AND IMBEDDING. 29 



If a large number of small pieces of tissue are to be imbedded, it 

 is often more convenient to imbed them in a small flat dish of suitable size. 

 The dish to be used is covered on its inner surface with a thin layer of 

 glycerin and partly filled with hard paraffin and the several pieces of 

 tissue to be imbedded transferred to it and arranged on the bottom of the 

 dish. As soon as a film forms over the paraffin the dish is placed care- 

 fully in cold water and the paraffin allowed to harden. The large piece 

 of paraffin thus obtained may then be cut into several smaller pieces, 

 each containing a piece of the imbedded tissue. 



On transferring an object from one fluid into another, so-called 

 currents of diffusion occur, which produce, especially in such tissues as 

 contain cavities, shrinkage and tearing. This often results in totally 

 changing~"the finer structure of the tissues. It is therefore necessary to 

 proceed with greater caution than in the method above indicated. Mix- 

 tures containing different percentages of alcohol and the intermediate 

 fluid (xylol, toluol, chloroform) may be prepared, and the object, ac- 

 cording to its delicacy, passed through a greater or smaller number of 

 such solutions. In ordinary cases a single mixture of alcohol and the 

 intermediate fluid is sufficient, the object remaining in the solution for a 

 length of time varying with its size before being passed into the pure in- 

 termediate fluid. This part of the treatment may of course be slowed or 

 hastened according to the number of such mixtures, each succeeding one 

 containing more and more of the intermediate fluid. After the object 

 has been passed into the pure intermediate fluid it should be just as care- 

 fully passed into the infiltrating fluid. If paraffin is to be used and the 

 object be delicate, the following method is advisable : The object is 

 placed in a glass vessel half filled with the intermediate fluid, into which 

 a few pieces of soft paraffin are dropped. The vessel is then covered 

 and allowed to remain at the temperature of the room. When the 

 paraffin is dissolved the cover is removed and the vessel placed in a par- 

 affin oven kept at a temperature corresponding to the melting point of 

 the paraffin. The volatile intermediate fluid evaporates gradually, and 

 in a few hours the object is infiltrated with an almost pure soft paraffin. 

 It may now be transferred into pure melted hard paraffin. In this the 

 tissue remains for a longer or shorter time, according to its size. 



It is often of advantage to infiltrate the tissues in a partial vacuum. 

 In this way there is obtained a better infiltration of the tissues with the 

 paraffin, and this seems to obtain a better consistency. Especially is 

 this method to be recommended in imbedding larger embryos or tissue 

 with cavities. A simple and convenient method is as follows : a glass 

 bottle of suitable size is warmed and partly filled with melted hard 

 paraffin and placed at one end of a copper plate, the other end of which 

 is heated by a flame, care being taken to heat the copper plate only 

 sufficiently to keep melted the paraffin in the bottle. The bottle is fitted 

 with a rubber cork with two holes, into which have been inserted two L~ 

 shaped glass tubes, provided, the one with a short rubber tube, which is 

 clamped, the other with a tube of sufficient length to reach to a Chapman 

 water-pump. The tissues are placed in the paraffin, the bottle tightly 

 corked, and the water-pump allowed to play for about half an hour, after 

 which the tissues are imbedded in the paraffin used during this procedure. 

 High temperatures are, as a rule, injurious to tissues. This 

 should always be borne in mind, and the student should aim to keep his 

 specimens at the lowest possible temperature conducive to proper infiltra- 

 tion. If for any reason higher temperatures become necessary, the ex- 



