384 Ralph S. Lillie 



Vejdovsky, Wilson and Beegh and more recently by Bürger 

 (1902). The iieplindia simply become gradually diff'eventiated out 

 of an origiually uniform and liomogeuous mesoblast which in early 

 stages presents no definite cell-boundaries. The differentiation of 

 the nephridium starts from the angle between septum and body- 

 wall; but at first no visible ditferences exist between the nephro- 

 blastic mesoderra and the adjoiuing myoblastic or septal mesoderm. 



The first signs of the appearance of the nephridium are visible 

 usually at the secoud septum in advance of the growiug zone 

 Young larvae of five somites thus as a rule exhibit the first larvai 

 nephridium in the earliest stages of its formation. The longitudinal 

 sectiou represented in Fig. 9, Piate 22 shows the differentiating 

 2*^ nephridium in somite V. At this stage the rudimentary organ 

 has the form of a somewhat triangulär mass of protoplasm con- 

 tinuous anteriorly with the fourth -septum. Fig. 10, Piate 23 shows 

 a similar stage in the formation of the fourth nephridium; and Fig. ol 

 similarly represents the rudiment of the sixth. The continuity with 

 the septum at the juuetion of the latter with the body-wall is per- 

 haps the most charaeteristic feature of the early nephridial rudiment. 



The histological ditfereutiation of the nephridial cells can he 

 studied to better advautage in transverse sections. In Piate 23, 

 Fig. 13, which represents a sectiou through the fourth septal region 

 of a larva of 5 — 6 somites, the somatopleuric cells of the veutro- 

 lateral region are entirely alike, with uothing to distinguish the 

 future nephridial region from the other portions of the mesoblast. 

 The protoplasm is homogenous and non-granular , the nuclei are 

 similar to one another in size and appearance, and cell-walls are 

 indistinguishable. At this stage in fact no essential differences exist 

 between the cells of this intersegmental region, and those of the 

 iutersegmental region of somites II and III, where no nephridium is 

 formed (Piate 23, Fig. 12). In Fig. 15, which represents a section 

 through the beginuing of somite VI in a larva of about eight somites, 

 one of the cclls of the right side is distinguished from the others 

 by the presence of a small space, bordered by deeply stainiog 

 protoplasm, which has macie its appearance in the immediate vici- 

 iiity of the nucleus. This minute space is the first indication of the 

 appearance of the uephridial lumen, which, it is of interest to note, 

 always seems to arise in the neighborhood of the nucleus. In other 

 respects the celi which shows this peculiarity is entirely similar to 

 the adjuining iiurely myoblastic cells, even to the possession 



