STUDIES IN SPICULE FORMATION. 273 
Clathrina cerebrum and contorta, in the former of 
which the gastral rays each possess two cells, and in the 
latter four cells, and in the huge monaxons of the majority 
of the Clathrinide, some of the more adult specimens of 
which possessing as many as five actinoblasts (Minchin) ; 
however, the precise development of these monaxons has yet 
to be determined. An illustration of (d) is to be found in 
such a genus as Heteropegma, in which the spicules situated 
in the cortical and medullary regions of the sponge differ 
largely as regards size, which difference in all probability 
solely results from the unlikeness of structural characters 
distinguishing these two regions of the sponge.’ In any given 
instance, the question as to how many of these factors are 
concerned, and in what proportion each has contributed to 
the result, can at present only be answered in a very general 
manner. 
Another subject for consideration is the relative numbers 
of monaxons and triradiates present in different sponges. 
As yet I have not sufficient data to draw any general con- 
clusions, and for the present I will only suggest a possible 
solution to the problem raised by the scarcity of monaxons 
found in many Clathrinide, as e. g. Cl]. contorta, in which 
the monaxons are very large. As pointed out before, asso- 
ciations of two cells are more lkely to occur than associations 
of three, and hence, merely on grounds of probability, the 
scarcity of monaxons in those sponges in which they occur at 
all is remarkable. Again, as previously remarked, sponges 
differ as regards the facility with which secretion occurs. In 
some sponges (as e.g. the Sycons above described) secretion 
occurs on division of a single scleroblast; in others, it is 
necessary that two cells should unite (as is apparently the 
case with Cl. contorta); and in others, no less than three 
cells must unite before secretion can take place (as in the 
majority of Clathrinide). Now, if we suppose that in Cl. 
contorta monaxons can be produced by union of two cells, 
but that the stimulus to secretion is very feeble, i.e. the 
1 This is a very doubtful instance. 
