STUDIES IN SPICULE FORMATION. 275 
more peripherally situated, are not adherent to each other, 
and hence are more adapted for centripetal migration. As 
to the question why the more peripherally situated division- 
products of the trefoil should incline gastrally rather than 
dermally, it can only be pointed out that in all probability 
the sextet, like the rest of the organism, is influenced by the 
pressures on the sponge-wall. The sextet will, for example, 
always tend to be so placed in the sponge-wall as that the 
plane of the trefoil cells shall be parallel to that of the wall 
on account of the pressures transmitted through the jelly of 
the wall from both sides,' and seeing that the trefoil portion 
of the sextet constitutes the ‘‘ body” of the cell-cluster, it is 
evident that all peripheral appendages (such as the future 
apical cells) will tend to swing to that side of it which is the 
less exposed to incident forces, i. e. gastrally—less exposed, 
since the pressures derived from the dermal surface of the 
sponge-wall exceed in intensity those derived from the gastral 
surface on account of a shield being provided by the opposite 
wall in the latter case. As the spicule becomes formed, the 
apical cell for the same reason will always incline gastrally at 
the extremity of the ray, the cell tending to rotate about its 
extremity (text-fig. 11), and this is probably the cause of the 
pyramidal conformation of the triradiate adapting it to the 
curvature of the sponge-wall.” Similarly the basal cells will 
also slide into the interspaces of the rays, in which position 
they are, as the figures show, normally found. In support of 
this conclusion may be named the fact that occasionally two 
cells are found in one interspace, both having slipped into 
the same retreat (figs. 23 and 24). 
Finally, a word as to the relations between the apical and 
basal cells in the triradiate spicule. At first, as we have 
1 T cannot call to mind having ever seen a “sextet stage” completely edge- 
on, though these of course may occasionally occur. 
2 The production of the “tripod spicules” in some species of Clathrina is 
perhaps to be explained in a like manner, the excessive gastral inclination of 
the three rays being due to the absence of opposing pressure derived from the 
gastral side of the wall, 
