MATERIALS FOR A MONOGRAPH OF THE ASCONS. 337 
unanimous testimony of all observers, including Woodland 
(1908), are produced, and built up entirely, each by a single 
cell. Woodland has also described typical monaxon spicules 
of certain molluscs as arising each in one cell (1907 [1]). In 
calcareous spicules we have some important analogies pre- 
sented by the formation of the gastral rays of the quadri- 
radiates. In considering this point there are two possible 
views to take with regard to the gastral rays; first, that they 
are sclerites of independent origin, not homologous at all with 
the monaxon spicules, in which case they do not furnish any 
analogy that can be used in my argument; secondly, that 
they represent, as I believe, monaxon spicules formed primi- 
tively in the interior of the oscular rim, which have become 
fused to the underlying triradiates. It may be asked, on 
this view, why monaxons formed on the dermal side did not 
also become fused to the triradiates. It is not easy always 
to suggest explanations for events that have occurred, but it 
is often still more difficult to explain why things have not 
happened that might possibly have taken place, and it can 
only be supposed that itis a more suitable arrangement for the 
monaxons that project on the exterior of the body-wall to be 
easily detachable structures, which are abandoned by their 
formative cells when completed. Now, if the homology of 
the gastral rays with monaxons be accepted, it is important 
to note that the gastral rays sometimes have one nucleus on 
them (Clathrina cerebrum), sometimes two, sometimes, as 
in Clathrina contorta, even four. On the ground of this 
analogy, as well as on more general grounds, I am inclined 
to think that the primitive sclerites from which the monaxon 
spicules of Calcarea arose were not originally formed between 
two nuclei, but by a cell with a single nucleus; and that the 
presence of two nuclei is a feature which was developed in 
correlation with the elongated, needle-like form, rather than 
antecedent to it.! 
1 Woodland explains away the difficulty, on his theory, of the single cell on 
the gastral ray by assuming a co-operation between the actinoblast of the 
gastral ray and the basal formative cells of the triradiate system. 
