Stracture and Development of tbe Nephridia. 377 



— the mesoblastic somites of tlie wliole larvai body being- indeecl 

 formed in au essentially unvarying manner. 



As the larva advauees in devel.)pment the nephridia of the more 

 anterior somites iuerease in size and underg-o a more complete 

 diöerentiation, at the same time as the others arìse in the growing 

 region. Horizontal sectious passing throngh the nephridial region 

 of such larvae may in fortunate preparations show the entire series 

 of nephridia, from the early undiffer.ntiated rudiraents of the region 

 adjoiuing the growing zone, to the well-defined nephridia of the 

 more anterior somites. Fig. 29, Piate 23 represents such a prepara- 

 tion in the case of a larva of about eleveu somites. On the right 

 side the section passes slightly ventrad to the ventral hooked setae 

 and shows the larvai nephridia from the first (in somite IV) to the 

 sixth (somite IX); the nndifi'erentiated rudiment of the 7*'^ (5^^ defini- 

 tive nephridium) is just visible at the angle between the newly 

 forming septum and tbe bodywall. In Figs. 16 to 22, inclusive 

 (Piate 23), bave been represented cross sections of the successive 

 nephridial somites and the post-mesodermal region of a larva in 

 which somite IX is in process of formation. The different stages in 

 the early development of a nephridium can thus be studied to 

 advantage in single larvae of this and similar stages. The develop- 

 ment of the nephridia is however so intimately bound up with that 

 of the somites that it will be necessary first of all to consider the 

 latter process in detail. 



The first changes, preparatory to somite-formation , that take 

 place in the posterior undififerentiated mesodermal region consist in 

 a multiplication and subsequent rearrangement of the mesodermal 

 cells, which at first are tightly packed together at the posterior angle 

 between the intestine and the body-wall (Plates 22 — 24, Figs. 7 — 10, 

 29 — 33). The manner in which this rearrangement is accomplished 

 may be seen from the above drawiugs, especially the sagittal 

 sectious (Piate 23, Figs. 10 and 31), and the horizontal sections 

 [Piate 22, Fig. 9 and Piate 24, Fig. 32). The closely packed posterior 

 mesoderm is bere seen to be bounded anteriorly by a roughly defined 

 transverse partition whose cells are directly continuous with the 

 tnesenchyme cells lining the cavity of tbe more fully formed adjoiu- 

 ing (somite VI, Fig. 9; somite VIII, Fig. 10). This partition, therefore, 

 represents the posterior wall of tbe somite next in front of the one 

 n process of formation, and defiues tbe position of the future inter- 

 ^egmental septum, of which structure it in faet forms the anterior 



25* 



