Structuie <and Development of the Nephridia. 343 



mg especial attention to a few i)eculiarities that bave hitherto 

 apparently eseaped Observation. 



2. Techiii(iue. 



Tbe adult nepbridia are best prepared for bistological study by 

 tixiug in Hermann's fluid for 15 to 30 miuutes, and then trans- 

 ferriug immediately to Merkel's fluid (AYihtman's modification, with 

 equal parts \^4 chromic acid and 0.25%' piatirne cbloride), where tbey 

 are allowed to remain for one to tbree bours. Tbey are tben wasbed 

 iu distilled water and transferred to alcobol as usuai. Tbe treat- 

 ment witb Merkel's fluid preveuts excessive blackening; tbe flxation 

 is apparently very faithful and tbe cilia are well preserved. Tbe 

 sections were mostly 7.5 /.i in thickness. Tbe reduced osmium may be 

 still furtber removed from tbe sections by leaving tbe slides over 

 night in a mixture of alcobol and bydrogen peroxide (usually com- 

 posed of about one part commercial H2O2 to tbree parts strong al- 

 cobol, mixed immediately before usiug). Tbe tissues are by tbis 

 treatment almost entirely freed of tbe osmium and left in a suitable 

 condition for stainiug. Tbe raost satisfactory stain was found to be 

 Heidenhain's iron baematoxylin, witb a counter-stain of erytbrosin. 

 For study witb oil-immersion objectives, tbe best mounting medium 

 proved to be tbickened cedar-oil (immersion oil). Tbis medium bar- 

 dens slowly, but bas tbe advautage of producicg a perfectly bomo- 

 geneous and transparent lacquer of great permanence and of almost 

 the same refractive index as that of tbe oil in wbicb tbe leus is 

 immersed. Very clear and sbarp images are tbus obtained witb tbe 

 highest powers. 



Tbe larvae were fixed by a great variety of metbods, of wbicb 

 only a few proved satisfactory. Treatment witb 1 f4 osmic acid for 

 2 to 5 miuutes, followed by Merkel's fluid for 1 — 3 bours, gave 

 fair preparations in mauy instances. Tbe best fixative for general 

 purposes proved to be Hermanx's fluid (2 — 5 minutesi, followed by 

 Merkel's fluid (l — 3 bours). In most instances, tberefore, tbe lar- 

 vae were fixed by tbe Hermann-Merkel metbod, and the sections 

 (mostly 5 ,u in thickness) were stained cbiefly with iron baematoxylin 

 and erytbrosin as above described, and mounted in cedar-oil. Very 

 clear and sbarply differenti ated prei)arations were tbus obtained. 

 Tbe tissues were remarkably well preserved, delicate structures 



Mittheüungen a. d. Zool. Station zu Neapel. Bd. 17. 23 



