20 ALFRED GIBBS BOURNE. 
Fig. 16 shows in a diagrammatic manner a late stage in the 
development of the secondary matrices in Urocheta. In all 
the five segments figured, except the most posterior, the primary 
matrix has completely separated into two secondary matrices. 
In the segments marked a and ¢ the outer secondary matrix is 
further dividing into two to place seta 4 in the very dorsal 
position it occupies in alternate segments. 
The investigation of the development of the secondary setal 
matrices from the primary ones in the Perichetes is attended 
with great difficulties. To work out the question thoroughly 
satisfactorily it would be necessary to obtain a series of very 
thin sections, so accurately transverse to the long axis of the 
embryo that in each segment one section passed through the 
whole of the region of the future seta ring. No amount of care 
would with certainty secure such a series, and my efforts in 
this direction have not been attended with any special luck. 
I have, however, sufficient evidence to prove that the only im- 
portant difference in this respect between the Perichztes and 
Moniligaster is in the number of secondary matrices which are 
produced. In the preparation shown in fig. 33, from the tail 
end of which the sections (figs. 1—8) tbat show the origin of 
the primary matrices were obtained, these primary matrices 
may be traced growing gradually outwards; older embryos 
show that they do this until they have grown right round the 
segment on each side. The cells meanwhile segregate to form 
the secondary matrices, the segregation commencing at the 
ventral ends of the matricial bands, and proceeding gradually 
towards the dorsal region. 
The later stages in the process of production of the secondary 
matrices I have most conveniently studied in Pericheta 
pellucida, but the small size of the embryo prevented my see- 
ing the earlier stages in that form. I think we may fairly 
assume, considering the close agreement that obtains between 
two such different forms as a Moniligaster and Mahbenus 
imperatrix, that the origin of the primary matrices and the 
proliferation of their cells to give rise to secondary matrices 
takes place as in those forms. The production of the secondary 
