Structiire and Development of the Nephrìdia. 403 



zone, and the diflferentiation of the seta-sacs and early septa are 

 shown. The proctodaeal cilia in tlie post-mesodermal region are 

 shown ; also the luesodermal nuclei applied to the proctodaeum and 

 body-wall of this region. X 800. 



Horizontal section through the posterior achaetous region of a more 

 advanced larva (ca. 1.4 lum. length ; ca. 45 somites) showing the growing 

 zone, and formation of early septa of this region. Post-mesodermal 

 region sharply defined; a valve separating proctodaeum and rectum; 

 cireular muscle-fibres applied to body-wall and anal region in this 

 region. X 800. 



Part of a horizontal section of a larva in which somite XIV is appearing. 

 Section passes tangentially to median ventral region a little to left 

 of nerve-cord. Shows early nephridia of somites V and VI {2^<^ provi- 

 sional and 1«* definitive nephridia), and their relations to septa, ventral 

 hooked setae and body-wall. X 800. 



Sagittal section of a larva in which somite VIII is forming; showing 

 early septa, incomplete dorsally. First septum distinctly separate 

 from retractor muscle of proboscis. Diaphragms have not yet appea- 

 red. X 300. 



Sagittal section of a larva in which somite XIV is forming ; showing 

 the series of septa and early stage of 2^<^ diaphragm. Remains of 

 3rd primitive septum seen stretching from Insertion of diaphragm to 

 anterior region of stomach. X 300. 



Horizontal section of left nephridium of somite VI (l^t definitive) in 

 larva of ca. 2 mm. and 50 somites, showing early condition of nephro- 

 stome, and its continuity with the septum. X 800. 

 Portion of a horizontal section through a more advanced larva than 

 the preceding (1.5 mm. long; 50 -t- somites); showing nephridium of 

 somite IX (fourth definitive), nerve-cord, and body-wall, cut tangentially. 

 Posteriorly the nephridium is fused with the ectoderm; no terminal 

 vesicle is as yet differeutiated. Continuity of the nephrostomial lip 

 with the primitive septum, and connection of the latter with the ventral 

 blood-vessel , are visible. The smaller cells at the anterior end or 

 the nephridium form the early nephrostomial epithel! um. X 800. 

 Sagittal section thiough nephridium and adjoining body-wall of so- 

 mite VI (Ist definitive nephridium) of a larva of about sarae stage as 

 last (ca. 45 somites; ca. 1.7 mm. long). Showing continuitj^ of septum 

 with dorsal lip of early nephrostome. The most anterior cells of the 

 nephridium are sraall and cubical; closely crowded nuclei in the por- 

 tion of the septum immediately continuous with the dorsal lip prob- 

 abl}' indicate a stage in the transformation of this portion of the 

 septum into funnel-cells. No dift'erentiated terminal vesicle as yet. 

 Nephridial blood-vessel not yet formed. X 800. 

 Cross section through posterior extremity of l^t nephridium (somite VI) 

 of a larva of a more advanced stage than that of Fig. 38 (ca. 2.25 mm.; 

 full number of somites), showing the posterior continuity of nephridium 

 and ectoderm and the presence of excretory granules in the latter. 

 No differentiated terminal vesicle as yet. The terminal region is 

 overarched bv a band of muscle-fibres continuous with the longitudinal 



