324 REAGENTS AND PROCESSES 



substance differentiated by immersing the sections for a short 

 time in a I per cent, solution of osmic acid. 



Paraffin. — The directions for imbedding material in paraffin 

 are given on page 261. Paraffin of about 52° C. melting-point 

 sections to good advantage at a temperature between 21° and 24° 

 C, or 70° and 75° F, Good cells for hanging-drop cultures 

 may be made by placing glass slides on the turn-table and spin- 

 ning rings on them by means of a camel's-hair brush dipped in 

 melted paraffin. 



Pepsin. — One part of pepsin-glycerine and 3 parts of water 

 acidulated with 0.2 per cent, of chemically pure hydrochloric 

 acid. When sections containing protoplasts are subjected to 

 this reagent at blood temperature, certain structures of the pro- 

 toplast which are insoluble in the reagent may be isolated from 

 those which are soluble. In the dividing nucleus the kinoplasmic 

 spindle-fibers persist after the chromosomes and nuclear plate 

 have been dissolved by this reagent. By the action of digestive 

 ferments on aleurone grains the ground substance is first dissolved 

 and then the crystalloid more slowly, while the limiting mem- 

 brane of the vacuole occupied by the aleurone grain persists. 

 Digestive ferments are thus found to be excellent reagents for 

 demonstrating the difference in constitution of the finer structures 

 of the protoplast and protoplasmic cell-contents. 



Phloroglucin. — This furnishes one of the most reliable tests 

 for lignin. Sections are placed in alcohol containing a trace of 

 phloroglucin, transferred to a drop of water on a slide and covered 

 with a coverglass. A drop of hydrochloric acid is then applied 

 to the edge of the coverglass and, as the acid comes in contact 

 with the lignified members, these are colored a bright violet-red. 



Phospho-molybdic Acid. — This is used as a test for pro- 

 teids. Sections are treated for an hour or two with a solution of 

 I gm. of sodium-molybdenum phosphate in 90 gm. of distilled 

 water and 5 gm. of concentrated nitric acid. Proteid materials 

 then take on the appearance of yellow granules. 



Picric Acid. — The structures of aleurone grains are well 

 differentiated by fixing in a concentrated alcoholic solution of 



