PREPARATION OF SECTIONS 105 



(chromic acid 2'5 gr., osmic acid TO, glacial acetic acid 1*0, water 

 1,000) ; or, bichromate of potassium 2 to 5 per cent. ; or, Miiller's 

 mixture (see p. 101), or chloride of platinum (1 : 300) ; or, chromic 

 acid and chloride of platinum (1 part each to 800 parts water). 



It cannot be said that these reagents are superior in any essential 

 particular to mercuric chloride. But, on the other hand, cases may 

 arise as for instance, where it is desired to produce certain con- 

 ditions, or for purposes of research beyond the limits of present 

 knowledge when other and newer methods must be employed. For 

 the beginner, however, the methods already described will suffice. 

 Bichromate of potassium may be found useful for preparing 

 specimens of organs, from Trematodes and Bothriocephales, which 

 contain yolk-glands and yolk-masses. This reagent colours the yolk 

 a deep brown, and a very good general effect is conveyed by specimens 

 prepared in this way. The specimens should be allowed to remain 

 in the stain for several hours ; they should then be carefully dehy- 

 drated, cleared, and mounted in Canada balsam. 



The preparation of Nematodes for section cutting is a more 

 difficult matter. It is scarcely necessary for the beginner to prepare 

 sections of the smaller varieties, as the more important details of 

 their construction are readily discernible in fresh material and in 

 specimens treated with glycerine. The large varieties should, how- 

 ever, be prepared in sections, the most instructive being Ascaris 

 lumbricoides. This parasite takes several weeks to harden. It should 

 be soaked in Muller's mixture, the fluid being frequently changed. 

 The specimen should then be rinsed in water, the process occupying 

 several days and the water being frequently changed. The specimen 

 should finally be put through the alcohol stages and, if these are 

 carefully managed, the worm will not shrink. A quicker method and 

 one which is also unattended by shrinkage of the object, is as follows : 

 First soak the worm in Miiller's mixture for a few days; then cut 

 into several pieces with a sharp razor ; allow the pieces to again soak 

 in the fixing mixture for a few days ; rinse in running water for 

 twelve hours ; and, finally, dehydrate by means of carefully graduated 

 alcohol stages. 



Sections may be cut by hand from the unenclosed specimen ; or 

 the material may be embedded in paraffin, celloidin, or soap. Nema- 

 todes treated by Looss's method with glycerine-alcohol, and finished 

 in pure glycerine, may be transferred to strong alcohol, cleared in 

 cedar-wood oil, and then embedded in paraffin preparatory to cutting. 

 The penetrative power of the different reagents will be enhanced if 

 fine incisions are made in the specimens. 



Other methods of fixing Nematodes recommended by various 

 authors are : mercuric chloride, both alone and in combination ; 



