No. 3.] THE EMBRYOLOGY OF THE EARTHWORM. 427 
nel of the succeeding nephridium. In favorable longitudinal 
sections this appearance may be followed from somite to somite, 
so that the series of nephridia appears to form a continuous 
cell-cord thrown into S-shaped undulations. On following the 
series forward, the group of cells marked s.g/. in Fig. 86, which 
lies between two succeeding nephridia, becomes enlarged, pro- 
jects into the coelomic cavity, is surrounded by a mesoblastic 
investment, and a seta is developed in its interior (Figs. 85 to 
88, and Fig. 93). The first setais soon followed by a second, and 
the two soon force their way through to the outside. The mes- 
oblastic investment persists and gives rise to the muscles of the 
sete. The setigerous glands of the inner series therefore 
grow forth from the nephric cords in regular alternation with 
the nephridia, and are accordingly of ectoblastic origin, being 
derived from the middle stratum of the germ-bands. 
The outer setigerous glands arise from the middle stratum of 
the germ-bands, lateral to the nephric cords, as solid ectoblastic 
invaginations invested by mesoblast precisely like those of the 
inner series. There can be little doubt that they arise from the 
lateral cell-cord where this is present, though I have been unable 
to demonstrate this, owing to the early disappearance of the 
line of demarcation between the lateral and nephric cell-cords. 
Several interesting questions suggested by these facts may be 
briefly pointed out at this point. If, as I suspect, the lateral 
teloblast be in reality a setiblast, an interesting side-light is 
thrown on the affinities of the Hirudinea, for the lateral teloblast 
of Clepsine is certainly homologous with that of Lusmdbricus, and 
its existence would seem to indicate the former possession of 
setae by these animals. It is an interesting fact, secondly, that 
the nephric cord is double, while the lateral cord is single, and 
it is not impossible that one of the so-called nephroblasts may 
in reality be a setiblast like the outer teloblast, though it is not 
possible to distinguish certainly the setigerous from the nephric 
elements of the so-called nephric cord until a comparatively late 
stage. In any case the origin of the setigerous glands, while 
showing a very remarkable and interesting specialized develop- 
ment, is entirely in accordance with the results of Kleinenberg, 
Vejdovsky and others as to their ectoblastic origin. 
