3gg Ralph S. Lillie 



It is uoteworthy tliat at the stage reprcsented in Fig. 34, 

 Piate 24 the two foremost pair of uephridia are in ali respects alike, 

 altlioiig-li tue anterior of the two later on completely degenerates 

 and disappears, while the posterior continues its development and 

 fornis the first definitive nephridiura of the adult. The first larvai 

 nephridium (in somite IV) has in most larvae already disappeared 

 at this stage. The time of its disappearance is variable; in some 

 larvae it has already disappeared at a stage of 9 — 10 somites, while 

 in otheis it may persist in a recognizable form until fifteen or more 

 somites bave beeu attained. The second larvai nephridium, which 

 correspoud to the first definitive nephridium of Ä. marina, also 

 degenerates and disappears, but not until a considerably later stage. 



(d) Later Development of the Nephridium. 

 The later development of the nephridium consists chiefly (1) in 

 an increase in size and an accompauying division into well-defined 

 cells with definite boundaries; (2) in a differentiation of the anterior 

 region of the nephridium, and apparently also of a portion of the 

 adjoining septum, to form the nephrostome, and in the establishment 

 of characteristic reiations with the blood-vessels; and (3) in the 

 formatiou of the terminal vesicle. Each of these processes will he 

 considered in order. 



Subdivision into Cells. 



In nephridia of the stage represented in Fig. 34, Piate 24 no 

 definite ccll-bouudaries are visible^ although the protoplasm has the 

 tyj)ical vacuolated structure and contains excretory granules. It 

 remains none the less continuous and undivìded from end to end 

 of the organ; its nuclei are small and as yet differ only slightly 

 from the neighboring mesodermal and eetodermal nuclei of the body- 

 wall. The lumen retains this intracellular character for a consider- 

 able period. Fig. 25, Piate 23 represents a cross section of the last 

 nephridium (somite XI) of a more advanced larva than that of 

 Fig. 34; the organ has increased considerably in size and its proto- 

 l)lasm, together with the largo nucleolated nuclei, shows the structure 

 characteristic of fully dififerentiated and functioual excretory cells. 

 As yet however, cell-limits bave not appoared and the lumen remains 

 intracellular. Fig. 37, Piate 24 represents a longitudinal section of 

 the correspoudiug nephridium (somite XI) of a slightly more advanced 

 larva in which the organ is beginning to show a subdivision into 



