STUDIES IN SPICULE FORMATION, 257 
not quite symmetrically placed about a common centre 
(a most improbable occurrence), there is no reason why the 
two nuclei most closely approximated should not, in virtue of 
their greater proximity, produce a monaxon, since, unlike 
what occurs in the case of the triradiate, the third nucleus is 
prevented from exercising a disturbing influence owing to 
the presence of a fourth nucleus, which is able to “ saturate” 
its ‘‘affinity,’” so to speak. In other words, granted the 
asymmetry of disposition, the four cells would pair off into 
two monaxon groups, and the potency, or “ affinity” of each 
cell would be satisfied. And similarly with an association of 
five scleroblasts, which, if it occurred, would probably resolve 
itself into a triradiate and a monaxon group. It will thus be 
seen that all these higher associations of scleroblasts 
differ from the trefoil in that in the former the 
“affinity” of each of the constituent cells can be 
immediately satisfied, whereas in the latter such 
is impossible, each of the three cells having to 
undergo division before secretion can occur. ‘This 
hypothesis of the ‘‘ saturation ”’ of cell “affinities ” thus not 
only readily explains the existence ot the three kinds of 
spicules, but also shows reason for the sole existence of these 
three forms. 
The production of a gastral ray on an already-formed 
triradiate basis is a phenomenon of a like order to the above. 
If in the oscular rim the central portion of a triradiate closely 
situated to the gastral layer comes into proximity to one ot 
the unspecialised cells composing the actively-growing epi- 
thelium of that region, then, two dermally-derived cells (one 
in the epithelium and one of the basal cells in an interspace 
of the triradiate—unless all three of the latter co-operate) 
being brought into apposition, the conditions essential to the 
production of a monaxon structure are fulfilled, and a 
monaxon disposed at right angles to the plane of the 
triradiate will be produced. It is true that the basal cells of 
the triradiate are already engaged in lime deposition, and 
hence are not free to co-operate with the future gastral 
