308 E. A. MINCHIN. 
that ig to say, which ultimately projects freely from the 
sponge-wall into the surrounding water. In most cases the 
free extremity of a monaxon spicule is marked by the pos- 
session of a so-called lance-head, a characteristic feature 
which is seen, when carefully examined, to be in reality a 
double bend in the shaft of the spicule, comparable rather to 
a bayonet than to a spear-head. The thickener remains 
attached to the head until this part has attained its full 
thickness and dimensions, but the founder travels away from 
the head, and lays the foundation of the shaft as a slender 
rod, which ends at first in the sharpest of points, and is far 
thinner than it becomes later (figs. 5, 6, 10O—12, 43, 78). . 
When the head of the spicule is fully formed, the thickener 
migrates slowly along the shaft towards the proximal ex- 
tremity to which the founder is attached, and as it travels 
down it builds up the shaft to its full, definitive thickness 
(figs. 7, 13—16, 79—81). No further increase in size is 
possible for that part of the spicule beyond which the 
thickener has passed in its course towards the proximal end. 
Thus the function of the thickener is to finish what the 
founder has begun. In many cases the activity of the 
thickener is indicated by a distinct rim, marking off the distal 
portion of the shaft, formed to its full thickness, from the 
proximal slender portion laid down by the founder alone (figs. 
18, 147, 79, 80, 87). As the distal extremity of the spicule 
is completely formed it begins to project from the dermal 
surface of the sponge, being pushed outwards from the body- 
wall. How this extrusion of the spicule is ‘effected is not 
quite clear, but it is perhaps due to the secretive activity of 
the founder-cell. The migration inwards (i. e. gastral-wards) 
of the founder in the early stages of the growth of the 
spicule soon brings it in contact with the bases of the collar- 
cells (see figs. 45—45, 71a, 71b). Beyond this point it cannot 
oo, and the subsequent growth in length of the spicule is not 
really due to any further migration of the founder in a 
proximal direction, but to a pushing out of the spicule in a 
distal direction. ‘Though the founder and thickener appear 
