18 ALFRED GIBBS BOURNE. 
and I put it forward with great diffidence and in a purely tenta- 
tive manner. 
Development of the Secondary Setal Matrices. 
The primary setal matrices grow in each segment laterally, 
and also to a lesser degree towards the ventral median line, as 
soon as they have taken up their position in the our and 
become covered by cceelomic epithelium. 
The primary matricial] cells become segregated to form the 
secondary matrices. The number of these formed in each seg- 
ment varies; in Moniligaster each seta couple arises from a 
single matrix, which accounts for the fact that the two sete of 
the couple are always so close together. This is also the case 
in Lumbricus. 
In Acanthodrilus sp. the sete are slightly “ separated,” 
and each seta develops from a separate secondary matrix. In 
Urocheeta there may be three or two secondary matrices on 
either side, according as the sete are “ scattered” or not in a 
particular segment on that side (fig. 16). The exact arrange- 
nent which obtains in regard to this matter in “Pericheta” 
pellucida is described below, but, speaking generally, in this 
form and in M. imperatrix and in Perionyx saltans each 
seta arises from a separate secondary matrix. 
All the secondary matrices on each side of the body remain 
connected together for a longer or shorter time by a band of 
tissue which stands out freely into the body-cavity, and is com- 
posed of the ccelomic epithelium cells which covered the primary 
matricial cells as they grew out to form the secondary matrices, 
and may be termed the intermatricial band (figs. 20, 24, &c., 
im.; cf. Vejdovsky, ‘ Entwickelungsgeschichtliche Unter- 
suchungen,’ Heft 3, Tab. xxiii, fig. 19, 7. f1, and Tab. xxviii, 
fig. 7, 7m.). 
Fig. 9 is a portion of an embryo of Moniligaster flattened 
out, drawn with a camera lucida and Zeiss BB, oc. 3, magnified 
about 150 diameters. At p. m. is seen a series of primary 
setal matrices, the more anterior ones commencing to grow 
outwards ; in the four segments (¢m.) the formation of the two 
