Stt-^icture and Development of a Horny Sponge. G7 



The lining epithelium of the embryo capsule is very 

 peculiar, and, so far as I am aware, entirely different from 

 anything which has hitherto been described in sponges. It 

 is composed of a single layer of enormous polygonal cells. 

 These cells, although flattened, are thick. In the youngest 

 embryo capsule they average about 072 mm. in diameter, 

 but up to 012 mm. in diameter in older ones, and they are 

 about 0024! mm. in thickness. The body of the cell is 

 finely granular, and each contains in its centre a very large, 

 flattened, oval nucleus, enclosing a number of deeply 

 staining granules. In transverse sections, the outer surfaces 

 of the cells are very often seen to be indented, and these 

 indentations would appear to correspond in some way to the 

 upper portions of the outer layer of cells of the embryo, 

 which in lite are closely connected with the epithelial layer 

 of the embryo capsule. In transverse sections also the body 

 of the cell is seen to be o-ranular throuohout, but the 

 granules are very much finer around the nucleus than 

 towards the periphery of the cell. I'lie cell always has a 

 very definite bounding wall on its outer and sometimes also 

 on its inner surface ; but frequently its inner surface, which 

 in life is pressed against the fibrous layer of the embryo- 

 capsule, exhibits no such wall. In sections the nucleus 

 sometimes appears solid and sometimes as a hollow vesicle 

 provided with a distinct wall and enclosing a granular 

 substance. 



These large epithelial cells very readily become detached 

 from the undeiiying fibrous layer of the capsule, and 

 sometimes remain adherent to the embryo when the latter is 

 removed from the mother sponge. 



Owing to its relationships 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. 



(3) The walls of the fiagellated chambers. — The walls ot 

 the fiagellated chambers are, of course, composed of collared 

 cells, but these cells exhibit certain ver}^ interesting details 

 in structure. 



Last year Professor Sollas showed* that in certain 

 sponges the collars of the collared cells (cho(inocytes) are 

 united together at their margins by a continuous membrane, 

 which forms a kind of inner lining to the chamber. I have 



* Article Sponges in the Encyclopaedia Britaunica. Ed. ix. 



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