5o8 FRESH-WATER BIOLOGY 



glycerine. This fluid is heated over a flame in a beaker or thin 

 watch glass until it begins to volatiUze, or more precisely to a 

 temperature of 56° to 60° C. The worms in a minimum amount 

 of fluid are dropped into the beaker, whereupon most forms 

 straighten at once. Specimens are preserved permanently in this 

 mixture and by allowing it to evaporate slowly one can bring them 

 gradually into strong glycerine in which they can be studied. This 

 method is especially good for mounting in to to. For histological 

 details nematodes should be killed in a mixture containing equal 

 parts of acetic acid, alcohol, and water, which has been saturated 

 with corrosive subHmate and to which has been added 0.25 per cent 

 osmic acid. 



Formol can be used to advantage only in the lactophenol quick 

 method. Nematodes are killed in 2 to 5 per cent formol and after 

 lying there 2 hours are gradually transferred to a solution com- 

 posed of I part glycerine, i part lactic acid, i part phenol, and 2 

 parts water. The transfer should be timed to bring them at the 

 end of 6 hours into the pure solution. 



Lactophenol specimens are mounted in the same fluid in a pre- 

 pared cell. Glycerine-alcohol material is mounted in strong glycer- 

 ine into which it has been carried gradually by evaporation. When 

 material must be stained and embedded for sectioning, or mounted 

 in balsam, treatment is very difficult and results are uncertain. In 

 general ah changes must be gradual and as deliberate as possible. 

 The simplest method is to employ a string siphon made by placing 

 three stender-dishes in a stair-step series, with the worms in the 

 middle dish and the fluid into which they are to be transferred in 

 the top dish while the waste flows into the bottom dish. String 

 siphons lead into and out of the center dish and the amount of the 

 flow is regulated by the size of the string. 



The differentiator (Fig. 811) is a very valuable aid in nematode 

 technique. Worms are placed in the small tube a and the tube h 

 is filled with the fluid into which they are to be transferred. The 

 very fine tip regulates the flow of the fluid. When in absolute 

 alcohol they can be taken out and brought into a clearing fluid 

 by the siphon method, or the differentiator may safely be used 

 by extending the fine tip e, and leaving out the mixing 



