A SIEVE-LIKE MEMBRANE IN LEUCOSOLENIA. 265 
not see it contract. But my observations are far from con- 
clusive on this point. The nucleus was best seen in prepara- 
tions stained with picro-carmine. In other preparations it was 
often not visible on account of the opacity of the cell. It was 
always spherical, lodged in the base of the cell, with a distinct 
cell membrane and nucleolus. In the living condition it was 
not visible. Besides vacuoles and a nucleus, the cells usually 
contained pigment granules, which appeared as black specks, 
both in the living condition and after osmic. They were 
especially distinct in sections made from sponges hardened in 
Flemming’s fluid and stained with safranin (fig. 22), which 
tinges them slightly. Sometimes there were very many granules, 
sometimes none at all. In the living cell these granules were 
observed to alter in position, but no regular direction of move- 
ment was noticed. Only two instances were found of what 
appeared to be collar-cells in division. In one of these (fig. 
18, e) no collars could be seen ; the flagella were very short, and 
no nuclei were visible (after staining with picro-carmine), but 
the whole cell appeared pinkish—from which, perhaps, it might 
be inferred that the nuclei were undergoing karyokinesis. In 
the other instance (fig. 18, d) two normal collar-cells were 
found joined at their base. The only noteworthy point about 
them was that each nucleus had two nucleoli, not in its centre 
but at opposite ends, as shown in the figure. I have never 
found, either in sections or maceration, any process connect- 
ing neighbouring collar-cells to one another, though I feel sure 
they must exist. 
Mesoderm.—In macerations I found the spicule to have a 
nucleus at the extremity of each ray, and a fourth at the con- 
fluence of the rays (figs. 15 a, 6). In some sections, especially 
those prepared with Flemming’s fluid followed by safranin, 
numerous irregular cells of a yellowish colour, and containing 
a number of black granules but no distinct nuclei, were to 
be found close under the collar-cells (fig. 22). These are 
perhaps identical with Topsent’s “‘cellules digestives.” I 
found no trace of muscular, elastic, or other special cells ; nor 
does there appear to be much occasion for them. 
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