STOLUX FOKM.Vl’lOX IX SPECIES OF TEYFAXOSYLLIS. 427 
the stolon. It is from this surface, of course, that the para- 
podia, witli their seta? and parapodial glands, will afterward 
develop, and the ventral ectoderm of the adult stolon is of 
much greater thickness and importance than the dorsal. 
These facts account for the early difference between the sur- 
faces. At first the ventral epithelium consists of a single 
layer of very deep cells, but in the succeeding stolons it 
becomes several cells in depth. 
The mesoblast of the interior of the stolon becomes applied 
to the epiblast of the ventral surface, though there is always 
a definite line between the two. But between the mesoblast 
and the dorsal ejiiblast there is from the beginning a space 
which becomes more marked as development proceeds. The 
next change which is noticed in the stolons is the segmenta- 
tion of the mesoblast (PI. 23 bis, fig. 14). From the ventral 
mass of undifferentiated material there grow out laterally and 
dorsally thin septa with a fibrous basis invested by globular 
mesoblast cells which become the peritoneal epithelium. 
This segmentation establishes itself very speedily ; while the 
mesoblast of one stolon is still undifferentiated that of the 
next oldest is almost completely segmented. Only at the 
anterior end, in the pedicle, and in the freely projecting 
posterior end does the mesoblast remain entirely unseg- 
mented. 
But while the segmentation of the mesoblast is thus shown 
in the formation of the septa and later the growth of the 
gonads, there remains at first a central mass of mesoblast, which 
gradually dwindles, however, until there is merely a central 
unsegmented longitudinal wisp of tissue — which occupies a 
dorsal position in the stolon — consisting of fibres with a few 
scattered nuclei (PI. 23 bis, fig. lb mes. r.). 
It is not for some time afterwards that the ectoderm begins 
to share in the segmentation, the parapodia being then really 
iormed by the pressure of the proliferating mesoblast beneath. 
But at this stage the setm form and the darkly staining gland 
cells appear in the epithelium (PI. 23 bis, fig. 15). 
In the development of the stolon, although the greater part 
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