eee 
—wv ver 
STUDIES IN SPICULE FORMATION. 259 
triradiate in its vicinity. Pore-cells are dermally-derived, 
and hence it happens that those pore-cells which happen to 
be situated in the neighbourhood of a triradiate are placed 
under conditions similar to those of the unspecialised cell of 
the oscular epithelium. The pore-cell itself being function- 
ally specialised, and necessarily bearing a one-sided position 
with respect to the centre of the triradiate with its three 
basal cells (in its three angles), its division is a necessity for 
the end to be attained. Although it is difficult to conceive 
how the division of the pore-cell is forcibly induced by the 
proximity of the basal cells of the triradiate, yet that such is 
the case can hardly be doubted. 
To sum up: a spherical isolated scleroblast gives rise to a 
spherical sclerite (especially well seen in Alcyonaria and 
Echinoderms) ; an elongated bi-polar (bi-nucleated) sclero- 
blast, according to the same laws of spicule-formation, gives 
rise to a monaxon; similarly, a trio of cells ultimately 
produces a triradiate structure; in short, the form of the 
spicule is evidently related to the form assumed by the 
secreting calcoplasm. Further, as the foregoing statements 
also prove, maintained apposition of two dermally-derived 
cells is in Calcarea essential to, and therefore the constant 
percursor of, the production of a monaxon spicule of appreci- 
able mass, and since this maintained apposition of cells occurs, 
apart from the instances just supplied, at only one situation 
in the ordinary calcareous sponge, it is only rational to 
attribute the gastral ray which is there produced to this 
cause. Granting two simple and easily verifiable propositions 
respecting the rationale of spicule-formation, it is thus 
possible to enunciate a theory consistent with the facts, or, 
at least, such as are at present known. 
Before considering the possible causes of the secondary 
forms assumed by spicules and other “features” of lime 
secretion, I will first discuss the disposition of the spicules in 
Calcarea, since the former will by this arrangement be more 
readily comprehended. 
