482 E. A. MINCHIN. 
To return now to this sphincter. It consists of two layers 
of fusiform ectoderm-cells arranged tangentially, between 
which one finds at intervals some of the large amceboid meso- 
derm-cells which occur throughout the sponge, but they are 
by no means common in the sphincter. The point I wish to 
emphasize is that the contractile muscular cells are the 
epithelial ectoderm-cells. 
The simple two-layered nature of the sphincter is apparent 
from the transverse sections 6 a, 6, c, 7 a, 8; but still more so 
from the tangential sections 6 d, 9 a, and c; less so in the 
thick section 7 6. Perhaps an even more convincing method 
of seeing it is to put a piece of the wall of a fresh living 
osculum into Ranvier’s one third alcohol for twenty-four hours, 
and then to carefully pull the sphincter off with a needle and 
examine it laid out flat in glycerine, after previous staining 
with picro-carmine. In such a preparation one sees, by care- 
fully focussing its surface, a layer of nuclei. If now the 
microscope be focussed deeper the layer of nuclei first seen 
vanishes, and a distinct layer of similar nuclei takes their place. 
There is absolutely no other cell layer but these two, unless 
one happens to find one of the scattered amceboid cells, which 
are by no means common. 
These fiat preparations offer the best means of studying the 
nature of the cells, and I find them differ on the opposite 
surfaces of the sphincter. On one surface they are spindle- 
shaped, elongated, and with distinct cell outlines. The 
spherical nucleus is surrounded by granules which form a 
fusiform figure, extending towards the two ends of the cell. 
Cells of this kind are shown in fig. 12. On the other surface 
the cells have similar nuclei, but no distinct cell outlines ; the 
granules are sometimes arranged in a fusiform figure, some- 
times not, but are much fewer relatively. Cells of this kind 
are shown in fig. 11. By focussing the preparation deeper 
from which fig. 12 was drawn, I could see cells similar to 
those represented in fig. 11; and similarly by focussing the 
preparation drawn in fig. 11, I could see cells like those in fig. 
12. I have tried to represent this point more clearly in fig. 
