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ARTHUR DENDY. 
I have no doubt that the wall of the oscular tube is completed 
on the inside by a delicate flattened epithelium, but I have 
not succeded in demonstrating its presence. 
(2) The Walls of the Embryo-containing Cavities. 
I have above stated the reasons for regarding the cavities in 
which the embryos are lodged as specialised parts of the exha- 
lant canal system. Whether this view be adopted or not — and 
it is still an open question — the structure of their walls maybe 
most conveniently treated of in this place, although I shall be 
obliged to refer again to this portion of my subject when 
speaking of the development. 
The only ovum which I have observed previous to the com- 
mencement of segmentation lies in a small cavity, about Cbl 
mm. in diameter, situate in the innermost part of the gela- 
tinous layer of the wall of an oscular tube. This cavity has a 
special wall, about 0'0144 mm. thick, composed of fibrous 
tissue with elongated nuclei, similar to that represented in 
fig. 15, but not so strongly developed. I have not detected a 
lining epithelium, but some of the nuclei which are observable 
in the outermost part of the wall may possibly belong to a 
delicate epithelial layer similar to that which lines the smaller 
branches of the canal system, and which must almost certainly 
occur here also. 
The large embryo- containing capsules now to be described 
are probably developed simply by growth of the small capsules 
containing the ova. The walls of these large capsules are, 
however, very much more highly differentiated than those of 
the ovum-containing capsules, and consist of two very distinct 
layers, (a) a fibrous layer, and (b) a lining epithelium. The 
fibrous layer of the wall (fig. 15) is very dense next to the 
lining epithelium, but further in it becomes looser and is 
broken into by large lacunar spaces. It is composed of cir- 
cularly arranged fibres each consisting of a greatly elongated 
fusiform granular cell, with a deeply-staining oval nucleus in 
the centre. The fibres are so densely packed in the outer part 
