STUDIES IN SPICULE FORMATION. B45 
of the future compound spicule, into which they develop by 
the further activity of the sextet cells, two of these being 
devoted to each of the three rays. Whether this deposition 
of three needles is preceded in each case, as in monaxons, by 
the formation of a vacuole or mould in the substance of the 
cell (bi-nucleated) is not easy to determine, though its 
probable occurrence is evidenced by a like condition occa- 
sionally to be seen in more advanced stages. A further point 
of some importance concerning this initial appearance of the 
triradiate system of deposits is the fact that each of these 
small needles is formed more in connection with the inner 
cell (i.e. the cell situated towards the gastral surface and 
which afterwards becomes the apical cell) than with the outer 
(cf. monaxons). Though both cells of each of the three pairs 
composing the sextet are essentially concerned in the pro- 
duction of the monaxon (it being probable, as pointed out 
below, that the monaxon or elongated form is in all cases 
determined by the presence of the two cells), yet this, in all 
probability, is for the greater part actually secreted by the 
future apical cell. The reasons for this supposition (for 
actual observation is difficult) are, as Minchin remarks, 
supplied both by the relations subsequently assumed by the 
two cells to the spicule-ray, and by the fact (which I have 
myself observed) that the nucleus of the inner cell stains 
more deeply at the initial stage of secretion than its com- 
panion—a reaction corresponding to functional activity. One 
more feature worthy of notice is the great preponderance as 
regards size of one of the three primary deposits in the sextet 
of S. ciliata (fig. 32)—a preponderance very slight and not 
at first apparent in the case of 8. coronata. This initially- 
larger needle generally, if not always, becomes the large 
vertically-disposed or “ posterior” ray of the adult triradiate 
spicule, although the young triradiate may not originally be 
so placed as to render this particular ray “ posterior ” in 
position from the first. Why one pair of sextet cells should 
thus form a needle so much larger than its two companions is 
a question at present not easy to answer. 
