44 CAUbAL GILLS OF /A'aOPfSiRID LARV^, 



Staining: — The gill-tissues, excepting perhaps the hypoderm- 

 layer, take up stains very slowh*. If a series of sections, running 

 from the rectal region of the abdomen into the basal third of the 

 gills, be arranged on one side, and stained in hiematoxylin, it 

 will be found that the body-sections become deeply over-stained 

 before the gill-sections are sufficiently stained to allow of difier- 

 entiation. Consequently, sections of gills require to be left in 

 the stain for a long time, until the hypoderm-layer is strongly 

 overstained. In the subsequent differentiation in acid alcohol, 

 the hypoderm may be allowed to remain slightly overstained, in 

 order to obtain the best differentiation in the internal tissues. 



Two double-stains may be recommended as almost equally good 

 for gill-sections, viz. :— 



(1) Ehrlich's Hjematoxylin with Eosin. The Eosin only stains 

 the cuticle and the endotrachese. 



(2) Heidenhain's Iron Hfematoxylin with Orange G. The 

 Hifematoxylin should be used as a purely nuclear stain, the 

 Orange G as a general cytoplasmic stain. The combination of 

 the two differentiates out the endotrachese and the outer layer of 

 cuticle in black, the iniclei in brown or greyish-black, and the 

 cytoplasm in dull orange. 



For the purposes of this paper, about 40 series of sections ha^ e 

 been prepared and stained by one or other of the above methods^ 

 Many of these sets are serial trans\ erse sections taken right 

 through from the region of the rectum to near the end of the 

 •nil, and thus running into more than one thousand sections of 

 10// thickness. Much labour may be sa^'ed by embedding the 

 three cut-off' ends of the gills close together in one block. In the 

 case of very long gills (e.g., Ani>-fro/fsfef<), the thi'ee cut-off' ends 

 may be arranged close together alongside the piece containing 

 the rectal region and the gill-bases, thus reducing the number of 

 sections to one-half. 



Nature of the Caudal Gills. 



In all Zygopterid larvae, the caudal gills are three in number, 

 viz., a single unpaired median gill, placed dorsally, and a pair of 

 lateral gilld, placed latero-ventrally, to right and left of the 



