STUDIES ON THE OOMPAEATIYE ANATOMY OP SPONGES. 341 
of the layer next to the lining epithelium that the outlines of 
the individual cells can no longer be distinguished, but further 
in the cells lie farther apart and the tissue partakes more of 
the nature of a compact stellate mesoderm. 
The lining epithelium of the embryo capsule (fig. 16) is very 
peculiar and, so far as I am aware, entirely different from any- 
thing which has hitherto been described in Sponges. It is 
composed of a single layer of enormous polygonal cells. These 
cells, although flattened, are thick, each one measuring from 
about 0072 mm. in diameter for the youngest embryo examined 
up to 0T2 mm. for older ones, and about 0’024 mm. in thick- 
ness. The body of the cell is finely granular, and each 
contains in its centre a very large, flattened, oval nucleus 
containing a number of deeply-staining granules. In the largest 
cells the nucleus may be seen to be undergoing division, 
doubtless preparatory to the division of the whole cell. Thus, 
in fig. 1G the nucleus of one of the cells has acquired a horse- 
shoe shape, the two arms of the horse-shoe being nearly 
separated from one another, and in another cell the division is 
complete and the cell contains two nuclei. I have observed 
no karyokinetic figures. 
In transverse sections the outer surfaces of the cells are 
frequently, but by no means always, seen to be indented 
(fig. 20). These indentations would appear to correspond in 
some way to the upper portions of the cells of the outer layer 
(ectoderm) of the embryo, which in life are closely connected, 
as wc shall see later on, with the epithelial layer of the embryo 
capsule. 
In transverse sections also the body of the cell is seen to be 
granular throughout, but the granules are very much finer 
around the nucleus than towards the periphery of the cells 
(figs. 17,21). The cell always has a 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 embrvo capsule, exhibits no such wall 
(fig. 21). 
In sections the nucleus sometimes appears solid (fig. 21), 
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