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ARTHUR DENDY. 
and sometimes as a hollow vesicle provided with a distinct wall 
and enclosing a granular substance (fig. 17). 
These epithelial cells very readily become detached from the 
underlying fibrous layer of the capsule, and sometimes remain 
adherent to the embryo when the latter is removed from the 
parent Sponge (fig. 18). In sections, owing to the treatment 
undergone, they very often appear entirely isolated, having 
been torn both from the embryo and from the fibrous layer 
of the capsule, or they may remain adherent to the embryo 
while separated from the fibrous layer, or to the fibrous layer 
while separated from the embryo. Owing to its relationship 
to, and intimate connection with, the outer layer of cells of 
the embryo (and for certain other reasons) I believe this peculiar 
lining epithelium of the embryo capsule to be nutritive in 
function, but to this point I shall return again when treating 
of the development. 
It must be borne in mind that there is no evidence actually 
to prove that these large epithelial cells belong to the mother 
Sponge and not to the embryo itself, but the latter hypothesis 
seems to me so improbable that I shall not consider it any 
further. 
(3) The Walls of the Flagellated Chambers. 
The walls of the flagellated chambers are, of course, com- 
posed of collared cells, but these cells exhibit certain very 
peculiar and interesting details in structure. 
In his article on Sponges in the ‘ Encyclopaedia Bi’itannica ’ 
(17) Sollas has shown that in certain Sponges the collar of the 
collared cells (or choanocytes, as he terms them) are united 
together at their margins by a continuous membrane which 
forms a kind of inner lining to the flagellated chamber. He 
says, “ In Tctractinellida, and probably in many other 
Sponges — certainly in some — the collars of contiguous choano- 
cytes coalesce at their margins so as to produce a fenestrated 
membrane, which forms a second inner lining to the flagel- 
lated chamber. The presence of this membrane enables us 
readily to distinguish the excurrent from the incurrent face of 
