Striictiire and Development of the Nephridia. 401 



Cross section thiough interseginental region 5 — 6 of a larva in which 

 semita VII is in process of formation, sliowing disposition and histological 

 structure of mesoblast in region of differentiating 6*^ septum. Longi- 

 tudiual muscle-fibres appeaiing. X ^00. 



Transverse section throiigh growing zone of a larva in which somite 

 VI is in process of formation. Section passes somewhat obliquely for- 

 wards from above down. Dorsally the undiflferentiated mesoblast is 

 shown; ventrally the region of formation of early 5*^ septum and 

 longitudinal muscle-fibres. Ectodermal cells large and well defined 

 with large nuclei. X SUO. 



Cross section through intersegmental region 5—6 in a larva in which 

 somite VII is just appearing; showing on the right side the commencing 

 ditferentiation of the third larvai nephridium (somite VI); lumen just 

 appearing in cells otlierwise similar to the adjoining myoblast cells. 

 Ectoderra cells well defined. On left side is shown early condition 

 of dorsal and ventral seta-sacs. X 8öO. 



Cross section through anterior of somite IV of a larva in which somite IX 

 is just appearing. 1«* provisionai nephridium shown in cross section. 

 Myoblast cells reduced. X 800. 



Similar cross section through somite V of the same larva. X 800. 

 Similar cross section through somite VI of same larva. X 800. 

 Similar cross section through somite VII of same larva. On left side 

 nephridium further advanced than on right. Muscle-fibres less developed 

 than in preceding soraites, and myoblast cells hirger. X 800. 

 Similar cross-section through somite VIII of the same larva. On the 

 left the section passes more anteriorly than on right and there shows 

 intersegmental region 7 — 8 with myoblasts. Nephridal cell not yet 

 distinguishable from the others. Shows also early seta-sacs which still 

 retain connection with the ectoderm and to which flattened mesoderm 

 cells are applied. Muscle-fibres less developed than in preceding 

 somite. X ^00. 



Section through region of formation of somite IX of the same larva 

 Ventrally the section passes through region of formation of 8^^ septum; 

 dorsally through the anterior border of the pigmented post-mesodermal 

 region. Large well-defined cells of the ectodermal growing zone. 

 Longitudinal muscle-fibres just making their appearance. X 800. 

 Cross section through post-mesodermal region of same larva, showing 

 small, pigmented, vacuolated, radially arranged ectoderm cells with 

 their small deeply staining nuclei. A few flattened mesoderm cells 

 applied to body-wall and proctodaeum. Strong cilia on ventral wall 

 of proctodaeum. X 800. 



Cross section through anterior region of somite XI of a larva in which 

 somite XII is in process of formation; showing commencing ditferentia- 

 tion of Sth larvai nephridium (üth definitive). Nephridium somewhat 

 more advanced in development on the left side. Myoblast cells well 

 defined but apparently undergoing reduction. Large embryonic nuclei 

 in ectoderm; smaller nuclei in the ventral region have the characters 

 of tissue nuclei, and are probably connected with the differentiating 

 nerve-cord. X 800. 



