274, W. WOODLAND. 
union of two cells only just suffices to ultimately initiate 
secretion, then it is easily understood how in such a case it is 
that such a very few monaxons are produced—the majority 
of the associations of two cells becoming associations of three 
cells in the long interval which elapses before the binary 
association can produce a monaxon. Such is a possible 
explanation of this and other like phenomena; whether it is 
the true one I cannot undertake to say. The fact, however, 
that, so far as I have been able to discover, in no cal- 
careous sponge do there co-exist small monaxons 
—monaxons presumably produced by division of a 
single scleroblast—with triradiates, but either small 
and large monaxons with triradiates, or solely large monaxons 
with triradiates, or triradiates alone, or monaxons alone, is 
suggestive and confirmatory of the idea just stated. Since, 
if a single binucleated scleroblast can produce a spicule, then 
the associations of two and three scleroblasts can; if secre- 
tion, however, only occurs on the associations of two and 
three scleroblasts, then only large monaxons and triradiates 
will result ; if further, secretion only occurs on union of three 
scleroblasts, then only triradiates will be produced ; if finally, 
secretion occurs very easily, then only monaxons will result. 
Why the two mother-scleroblasts which produce Clathrinid 
monaxons undergo division to such an extent as to give rise 
to such huge structures, it is impossible as yet to say. 
One more explanation may be provided. It has been seen 
in Part I of this paper that the inner cell (next the gastral 
layer) of each of the three pairs of cells constituting the 
sextet is chiefly concerned in the formation of the ray, and 
that the outer cell of each such pair is basal in position, and 
with its two companions constructs the central portion of the 
triradiate system. Why should there be this particular 
division of labour? The answer is doubtless largely to be 
found in the fact that the cells constituting the trefoil are 
from the first central in position, and closely adherent to 
each other, and therefore tend to maintain this disposition, 
whereas the three division-products of these are naturally 
