PLANT AND ANIMAL SECTIONS 259 



more complete sections. Animal and delicate plant 

 tissues must be imbedded in paraffin to enable thin 

 sections to be made. The paraffin wax employed should 

 have a melting-point of from 45 to 52 C., and be kept 

 melted in a water bath, the temperature of which should 

 not be allowed to rise more than one or two degrees above 

 the melting-point of the wax. 



Imbedding in Paraffin. The material, hitherto pre- 

 served in 70% alcohol, is dehydrated by washing in 90% 

 alcohol and transferring to absolute alcohol for a period 

 varying from 1 to 12 hours according to its size, the 

 alcohol being changed once or twice in the meantime. 

 It is then placed in cedar oil or xylene until translucent, 

 then immersed in melted paraffin until thoroughly 

 impregnated. This may take from 1 to 6 hours. The 

 object is transferred to a watch-glass and melted wax 

 poured over it, and then rapidly cooled by floating on 

 water. When the wax is quite cold it is ready for 

 cutting. 



An alternative method that is generally adopted for 

 plant material is as follows : 



The dehydrated material is placed in a mixture of 

 absolute alcohol and chloroform, and then in pure 

 chloroform until cleared. Then placed in a saturated 

 solution of paraffin in chloroform for some hours, after- 

 wards transferred to melted wax until all traces of 

 chloroform have been driven off and the object is 

 thoroughly impregnated with wax, finally imbedded in 

 a watch-glass as previously described. 



Cutting and Mounting Sections of Imbedded Material. 

 A microtome is necessary for cutting imbedded sections ; 

 very good results can be obtained with one of the less 

 expensive kinds such as Cathcart's, but for serial 

 sections one of the more elaborate forms, such as the 

 Minot or the Cambridge rocking microtome, is essential. 

 For large objects, as, for example, longitudinal sections 



