INTRODUCTORY. ' 6 



increasing strength, final dehydration vvitli absolute alcohol, 

 imbibition with an essential oil or other so called clearing 

 agent which serves to remove the alcohol, and lastly either 

 mounting at once in balsam or other resinous medium or 

 imbedding in paraffin for the purpose of making sections. 

 The dehydration method is the course which is generally 

 preferred, chiefly because of its great superiority as regards 

 the preservation of tissues. For the presence of water is 

 the most important factor in the conditions that bring about 

 the decomposition of organic matter, and its complete removal 

 is the chief condition of permanent preservation. 



3. Dehydration. The further course of preparation by the 

 dehydration method is as follows : At the same time that 

 the superfluous fixing agent is being removed from the tissues, 

 or as soon as that is done, the water of the tissues must be 

 removed. This is necessary for two reasons : first, in the 

 interest of preservation, as above explained ; and secondly, 

 because all water must be removed in order to allow the 

 tissues to be impregnated with the imbedding material 

 necessary for section-cutting, or with the balsam with which 

 they are to be finally preserved. This dehydration is per- 

 formed as follows : The objects are brought into weak 

 alcohol, and are then passed through successive alcohols of 

 gradually increased strength, remaining in each the time 

 necessary for complete saturation, and the last bath consisting 

 of absolute or at least very strong alcohol. 



In dealing with delicate objects, it may be necessary to take special 

 precautions in order to avoid injury to them through the violent 

 diffusion-currents that are set up in the passage from watei* to alcohol, 

 or from one bath of alcohol to another of considerably different density. 

 Some kind of diffusion-apparatus may conveniently be used in these 

 cases. The objects may be placed with some of their liquid in a tube 

 corked at one end and closed at the other by a diaphragm of muslin or 

 chamois skin or other suitable membrane, the tube being then immersed 

 in a vessel containing the grade of alcohol that it is desired to add to 

 the liquid in the tube, and the whole allowed to remain until by 

 diffusion through the diaphragm the two liquids have become of equal 

 density. Or, COBB'S differentiator (Proc. Linn. Soc., N.S.W., v, 1890, 

 p. 157; Journ. Roy. Mic. Soc., 1890, p. 821) may be employed. Or, the 

 apparatus of HASWELL (Proc. Linn. Soc., N.S.W., vi, 1891, p. 433; 

 Journ. Roy. Mic. Soc., 1892, p. 696). Or that of CHEATLE, described in 

 Journ. Pathol. and Bacteriol., i, 1892, p. 253, or Journ. Roy. Mic. Soc., 



