402 Ralph S. Lillie 



Fig. 24. Cross section through intersegiuental region 11 — 12 of a larva in which 

 soinite XII is appearing; showing early mesoblast; intestine surrounded 

 by blood-sinus coutinous with dorsal and ventral blood-vessels; large 

 well-defined ectoderm cells of the growing zone. Diflferentiation of 

 the ventral nerve-cord is in progress, and smaller, more deeply staining 

 nuclei are seen in its vicinity. X 80Ü. 



Fig. 25. Cross section through nephridium and adjacent body-wall of somite XI 

 of a larva of ca. 1.5 rum. length and ca. 45 somites. Nephridium (6*^ 

 definitive of right side) stili apparently one celi in cross section ; 

 showing also a peritoneal fold runniog along nephridium from septum 

 in a Position corresponding to the later nephridial blood-vessel. 

 Nucleus large and nucleolated. Protoplasm vacuolated and containing 

 granules; stains more deeply at border of lumen. Cross-cut cilia 

 visible in lumen. X 800. 



Fig. 26. Cross section through nephridium and adjacent body-wall of somite Vili 

 of a larva of a later stage than that of Fig. 25 (ca. 1.6 mm.; ca. 50 

 somites). Nephridium (S^d definitive of left side) has become divided 

 into cells and lumen has become intercellular ; 3 cella present in cross 

 section at this region. Histological characteristics of cells same as in 

 Fig. 25. A similar membrane occupies position of future nephridial 

 blood-vessel. 



Fig. 27. Cross section through nephridium and adjacent body-wall of soraite VIII 

 (3r<i definitive nephridium, left side) of a more advanced larva than 

 that of Fig. 26 (ca. 2,0 mm.; full number somites); section passes just 

 behind nephrostome and shows nephridial blood-vessel with an early 

 germ cell applied to it. X 800. 



Fig. 28. Similar cross section through middle region of Ist definitive nephri- 

 dium (somite VI) of right side of a larva of later stage than that of 

 Fig. 27 (2.5 mm.; full number somites). Nephridium 5 cells in cross 

 section; histological structure as above. Nephridial blood-vessel 

 present in anterior portion of nephridium. X 800. 



Fig. 29. Horizontal section through posterior body-region of a larva in which 

 somite XI is being formed. Section passes along level of nephridia 

 and ventral setae, and shows the entire series of early nephridia. 

 X-IUO. 



Fig. 30. Sagittal section through posterior body-region of a larva in which 

 somite XI is being formed. Shows early septa, intestine, nerve-cord. 

 Septa in advance of somite VIII are dorsally incomplete. Shows also 

 large terminal mesoblast-nucleus. X ^00. 



Fig. 31. Longitudinal section through early region of formation of nephridia 

 in a larva in which somite XI is just appearing; showing early septa 

 and associated nephridial rudiments of the C^h and 7*^ larvai nephri- 

 dia (somites IX and X). Pole-nucleus of mesoderm and raitosis in un- 

 differentiated mesoderm cells also seen. Also the large ectodermal 

 nuclei of the growing zone. X 800. 



Piate 24. Arenicola cristata. 



Fig. 32. Nearly horizontal section through the posterior region of a larva in 

 which somite XVI is in process of formation. The ectodermal growing 



