272 W. WOODLAND. 
being the path of least resistance! (see text-fig. 10 above). 
And, moreover, besides the constancy of direction of the 
transmitted pressures which affect the spicule of the adult 
sponge as contrasted with the indefiniteness of the conditions 
prevailing in the sponge larva, it must be remembered that 
the volume of the gelatinous matrix in which the spicule is 
embedded is considerably reduced in the adult sponge, and 
hence there is less space for the deviations from the typical 
triradiate form to occur in—the slight heterogeneities of 
constitution of the jelly cannot produce an appreciable effect. 
There is thus not only in the adult sponge a positive set of 
conditions tending to produce the regular triradiate structure, 
but also conditions which tend to negative asymmetry of form.” 
Again, spicules vary enormously as regards size, and these 
differences are as evident in the same sponge as in different 
individuals. The factors responsible for the size of a given 
type of spicule seem to be (a) the constitutional efficiency of 
the scleroblast; (b) the initial number of scleroblasts con- 
cerned in the production of the spicule; (c) the amount of 
fission which the actinoblast undergoes during the formation 
of the spicule; (d) the character of the region of the sponge 
in which the spicule is situated; and (e) the nature of the 
external environment, this partly depending upon the situa- 
tion of the sponge in regard to its immediate surroundings, 
and partly upon the distribution of the species. Illustrations 
of (a) are to be found in every individual sponge, and in the 
different species and genera. As an illustration of (6) the 
difference between the two kinds of monaxons in Sycon 
coronata and ciliata already described may be instanced. 
Illustrations of (c) are to be found in the gastral rays of 
1 A thin sponge-wall with its contained spicules may, in connection with the 
movements of the surrounding water, be likened to a lattice-work with a sheet 
spread over it on the side next the wind—the sheet, like the sponge-wall, 
bulging through the interspaces. 
2 See my paper on the spicules in Aleyonium digitatum (Study IT). 
It is a rule holding for calcareous spicules generally that localisation in a 
matrix (always more or less heterogeneous in constitution), apart from any 
limiting layer, always tends to indefiniteness of form. 
