16 ALFRED GIBBS BOURNE. 
antero-posterior position of those three sections. Fig. 1 passes 
through the “ primary mesoblast ” of the right side; the next 
section on one side (not figured) is precisely median and passes 
between the “ mesoblasts,’’ while the section beyond (not 
figured) passes through the “ mesoblast” of the left side. 
Fig. 2 is the next section to the right of fig. 1, and fig. 3 the 
next beyond; they show the row of cells proceeding from the 
“‘mesoblast”’ of fig. 1 passing slightly outwards and forwards 
(cf. fig. 33). Fig. 4 is a portion of the section next to fig. 3, 
and commences about half its own length beyond where fig. 3 
leaves off ; it shows the row. of cells proceeding from the neuro- 
blast of the right side. The lines 4 B in figs. 4, 5, and 6 are 
drawn at the same level to show the relative antero-posterior 
positions of the figures. Fig. 5 is the seventh section from 
fig. 1, but the cells a, 6, and c are inserted from the eighth and 
ninth sections, while fig. 6 is the tenth section and the most 
lateral of the series. 
Figs. 5 and 6 show the origin of the primary setal matrices ; 
they arise on each side from a longitudinal row of cells which 
is, I have little doubt, the row arising from Wilson’s lateral 
teloblasts.! These cells, at first very superficial, take up as 
they pass forwards a deep-lying position within the cclom. 
At first there is one cell on each side in each segment lying 
close to the nephridial cell (fig. 5). Their further history is 
shown in figs. 7 and 8; fig. 8 comes from near the median line 
a little in front of fig. 4, and includes a portion of the nerve-cord ; 
fig. 7 lies a little behind and to one side of fig. 8. The setal 
matrices are now seen to consist of several cells. There is not 
the slightest difficulty in tracing these structures forwards 
from segment to segment until they are old enough to have 
developed sete. 
The embryos of Moniligaster (probably M. sapphirinaoides) 
which I have in these early stages show also very clearly that 
the primary setal matrices take their origin from continuous 
longitudinal rows of cells, but neither here nor in Mahbenus, 
at any rate in the youngest stages which I have examined, are 
1 «Journal of Morphology,’ vol. i, 1887. 
