484 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS 



have been successfully disposed of, the cell may be ' sealed ' and 

 ' ringed ' in the manner already described. 



Preparation of Soft Tissues, It is impossible in the limited 

 space at disposal here to do more than give a sketch of the very 

 elaborate art of histological preparation. The reader who desires to 

 pursue the subject further will find all necessary information in Mr. 

 A. Bolles Lee's ' The Microtomist's Vade-mecum ' (London : J. & A. 

 Churchill), from which work the information here given is for the 

 most part abridged (the passages in quotation marks in the following 

 pages are taken therefrom verbatim). 



Fixation. ' The first thing to be done with any structure is to 

 fix its histological elements. Two things are implied by the word 

 * fixing : ' first, the rapid killing of the element, so that it may not 

 have time to change the form it had during life, but is fixed in 

 death in the attitude it normally had during life ; and second, the 

 hardening of it to such a degree as may enable it to resist without 

 further change of form the action of the reagents with which it may 

 .subsequently be treated.' For instance, if you were to take a living 

 rotifer and throw it into one of the usual staining fluids or preser- 

 vative liquids, it would at once contract into a shapeless mass, the 

 elements of its tissues would be neither properly stained nor properly 

 preserved, and the result would be an unrecognisable caricature of 

 the living organism. But if it be first properly killed and slightly 

 hardened in the proper manner, it may be permanently mounted in 

 .such a way as to show, uninjured and undistorted, even the most 

 delicate details of its structure. 



Fixation is generally performed by immersing the object to be 

 fixed in an appropriate liquid, and leaving it therein until the 

 desired degree of hardening has been obtained. After that the 

 object is well washed to remove all excess of the fixing liquid. The 

 object may then be further prepared by the wet method, in which 

 all subsequent operations are performed by means of aqueous media. 

 It may be mounted at once in an aqueous mounting medium, or it 

 may be stained (see below), or it may be put away till wanted, with- 

 out mounting, in some preservative medium. 



Or ' the object may be further prepared by the dehydration 

 method ' (see below), * which consists in treatment with successive 

 alcohols of gradually increasing strength, final dehydration with 

 absolute alcohol, clearing ' (see below) ' with an essential oil or other 

 clearing agent, and lastly either mounting in balsam or imbedding 

 in paraffin for the purpose of making sections/ 



Corrosive sublimate is the fixing agent that is most to be recom- 

 mended for general work. A good formula consists of a saturated 

 .solution in water containing 1 per cent, of acetic acid. The present 

 writer adds a little nitric acid, say 1 per cent., which helps to 

 make the solution keep without precipitating. Another good solu- 

 tion is a saturated solution in alcohol of 50 per cent., or even 70 per 

 cent., also with addition of 1 per cent, of acetic acid. 



Whatever solution is taken, the objects should be removed from 

 it soon after they have become thoroughly penetrated by it. For 

 .sublimate hardens very rapidly, and makes tissues brittle if they are 



