STUDIES IN SPICULE FORMATION. 955 
trefoil, so fulfilling this last obligation, it is impossible as yet 
to say. Nor is it possible to supply a definite answer to the 
question as to why it is that a triangular system of three 
monaxons is not produced; it can only be pointed out that 
We possess no evidence that a nucleus can, in Calcarea, 
stimulate secretion in two places at once, and that the 
trefoil stage tends to show that the nucleus does not possess 
such a capacity. In actuality, deposition does not occur 
until each constituent cell of the trefoil has divided, and then 
the three monaxons produced inevitably tend, from the 
initial triradiate construction of the trefoil, to form a tri- 
radiate system. As in the case of the monaxon, the 
triradiate form is directly related to the conforma- 
tion of the secreting agency. I am aware that this 
interpretation of the form of the triradiate is disputed, but 
until it is clearly shown how, e.g. surface tension can by 
itself ‘“‘lead to the growth of three actines inclined at angles 
of 120° to each other” (Sollas), or how pore-distribution can 
effect the same result when the pores are absent (as in sponge 
larvee), I must adhere to the explanation I have provided. 
A simple explanation which, as will be seen, simultaneously 
explains the conformation of all three types of -spicule— 
monaxon, triradiate, and quadriradiate—has, | think, some- 
thing to recommend it.! 
- 1 The objections urged against this proposition, viz. that the triradiate 
form is directly correlated with the conformation of the sextet are, in the 
words of Prof. Minchin, as follows: “The actinoblasts are never exactly 
equal, or perfectly regularly placed, nor are the rays formed exactly in the 
axis of the cell, but almost always a little to one side or the other; hence, if 
that were the only factor at work, we should rather expect irregularity to be 
the rule and equality between the angles to be a rare exception.” But it is 
evident from this statement that Prof. Minchin does not really dispute the 
proposition that the mere triradiate form owes its origin to an association of 
three cells (in the same way that a monaxon is due to the presence of two, 
or a spherical spicule results from one), since if, among the triradiates, 
irregularity were the rule and ‘‘equality between the angles... a rare 
exception ” he would readily accept it; the real objection of Prof. Minchin is 
to the minor dectrine, viz. that the mere association of three cells is sufficient 
to account for the extreme regularity (equirayed and equiangular) of the 
