244 : W. WOODLAND. 
pretty frequently (figs. 15, 16), small size of nuclei denoting 
recent cell-division, each cell of the trefoil subsequently 
dividing to form the “sextet”? soon to be described. It is 
well to mention that nuclei only in the same individual 
sponge can be thus compared as regards size, since this 
character appears to vary slightly in different specimens, and 
certainly does in the two species, the nuclei of 8. ciliata 
being much smaller than those of S. coronata. Three cells 
thus approximate to constitute the trefoil, and the more 
advanced the stage, the more closely associated are the cells. 
It is difficult to say exactly what this association consists of 
—whether, as seems probable, the cell-edges actually fuse, or 
whether they merely come into contact—but, however this 
may be, the three cells come together in much the same way 
as three billiard-balls would, and consequently possess the 
same triradiate formation thus conspicuous from the very 
beginning. Each of these constituent cells in the mature 
trefoil then divides centripetally and approximately radially 
in a direction more or less inclined towards the gastral 
epithelium, so forming the sextet, or six-cell stage, in the 
development of the triradiate spicule (Pl. 13, fig. 17, and 
Pl. 14, fig. 31). 
It must here be pointed out that in the figures given depth 
of coloration of the nuclei is an artifice adopted to indicate 
relative elevation (high focus), or, in other words, proximity 
to the gastral surface from which the preparations are viewed 
(cf. nuclei of apical and basal cells) ; the depth of coloration 
actually observable in the preparations denotes, as remarked 
below, functional activity, and bears no relation whatever to 
the position of the nuclei. The device must be carefully 
distinguished from the fact. 
The next advance from the sextet stage is the deposition 
in three centres of small calcareous elongated masses, needle- 
like, as in the case of the monaxons, and radially disposed, 
so as to include three equal angles at the centre of the cluster 
of six cells (fig. 18 for 8. coronata, fig. 32 for S. ciliata). 
These three at-first-separate needles constitute the rudiments 
