17 CUTTING AND MOI'\T1.\<; TISSUES. 



melted hard paraffine and cooling it quickly by immersing in cold 

 water. The paraffine block can then be placed in a microtome and 

 the object sectioned. This method will apply only to those objects 

 that will not be shriveled by the high temperature of the melted 

 paraffine. 



For thorough, systematic study some method should be used 

 that will enable sections to be made with accuracy, and if need be, 

 in a series with a known definite thickness. The method should 

 also provide something that can be infiltrated into the tissue and 

 hardened to prevent crushing while being cut. The collodion and 

 paraffine methods are the ones usually used for this purpose. 

 Many modifications of both have been suggested but each method 

 will be outlined to apply to the more general kinds of tissues. 

 Experience will assist in modifying them for special cases. 



Paraffine Hethod. 



Much discussion has arisen in regard to the relative merits of 

 the different paraffine methods, but the general differences are 

 more or less of an unimportant nature and because of the dif- 

 erent kind of tissues subjected to treatment. The most im- 

 portant methods are those of Mohl, (Bot. Gazette, Jan., 1888), and 

 Schoenland (Bot. Gazette, July, 1887), while most of the others are 

 modifications of these resulting from the experiments of different 

 workers on the different classes of tissue. 



The tissue to be treated is first hardened in chromic or picric 

 acid or mixtures of these with other agents. As was suggested by 

 Mohl, the acids act on the tissues in some way and make them 

 much more penetrable to the infiltrating mass. Paraffine does not 

 easily penetrate tissue treated with alcohol, yet many recommend 

 it as a good hardening agent even under these conditions. A good 

 mixture for hardening is made from equal parts of 1 per cent, chro- 

 mic acid, osmic acid, 2 per cent., and acetic acid, 1 per cent. The 

 osmic acid is very useful in many hardening agents for fixing the 

 protoplasm, especially if it is desired to demonstrate karyokinesis, 

 or cell division. Tissue should be kept in this mixture from 24 to 

 48 hours and then washed with running water 6 to 8 hours, after 

 which it is placed for 12 hours successively in alcohol of 20 per 

 cent., 40 per cent., 60 per cent., 80 per cent., and 95 per cent., 



