STUDIES IN SPICULE FORMATION. 317 
triradiate spicules (only one of which persists as a part of 
the mature larval skeleton, the others becoming gradually 
absorbed) are developed from the several granules initially 
deposited in the longitudinal strand of cells (figs. 17, 20, 
22), and if the single spicule possesses a triradiate mould 
in which to deposit its substance, so also must the several. 
From these facts, then, it would seem that, instead of the 
mould determining the form of the spicule, the contrary is 
rather the case, since the number of moulds is determined by 
the number of spicules. The elimination of this factor of a 
mould leaves us the disposition of the secreting cells as the 
only known cause to which we can attribute the triradiate 
conformation. We must imagine that the cells extending in 
any one direction from the centrally-situated granule or 
granules co-operate to exert a species of tractive influence on 
the freshly-deposited lime, tending to cause this to be 
deposited in a direction which is that of the resultant of 
their individual “ pulls,”’ and which the cells eventually place 
themselves in line with: such a conception, indefinite though 
it may be, seems at present alone capable of accounting for 
the facts. Compare the development of a triradiate spicule 
in a calcareous sponge (16, 24). Here the triradiate, unlike 
that of the pluteus, originates in three centres—in three 
pairs of cells, three cells of the three pairs being closely 
apposed centrally (the ‘“trefoil’’), and three being more 
distally situated—and each of the three lime deposits is, 
owing to the fact that it originates in one cell with another in 
close apposition, elongated from the commencement—the 
influence of an adjoining cell is obvious. Again, in the 
sponge triradiate, owing to each deposit, i.e. each ray, having 
only one cell in its vicinity in addition to that in which it 
originates, this is fairly straight; on the other hand, in the 
pluteus the initial deposit has, in any one of the three 
directions, not one, but many cells in its vicinity, and the 
resulting ray is correspondingly curvilinear. It will be seen 
from what has just been stated how impossible it would be to 
assume that in the pluteus triradiate the direction of a given 
