STUDIES IN SPICULE FORMATION. Pay he | 
THe PHYLOGENETIC EVOLUTION OF THE SPICULES IN CALCARBA. 
The theory here adopted as to the origin of calcareous 
spicules has already been implied in the foregoing; in other 
words, the phylogenetic process is here held to be, in all 
essentials, identical with the ontogenetic process. It will 
tend to still further enforce the general argument if I here 
give a brief resumé of the above discussion from the present 
point of view. 
Granting the presence of numerous isolated scleroblasts in 
the substance of the sponge-wall (generally in regions of 
vigorous growth), the capacity of their cytoplasm to secrete 
hme, and the more or less fortuitous association of these 
scleroblasts in twos, threes, and higher aggregates, and, as 
shown above, it is not only possible to show why monaxons, 
triradiates, and quadriradiates have been produced, but also 
why, in the normal course of things, spicules of other forms 
have not. I have further attempted to show that the modes 
of disposition and secondary forms of the spicules are in- 
evitable results of environmental influences operating during 
the course of each individual ontogeny. Thus, e. g. as regards 
the mode of disposition which I have termed “ oscular,” we 
have initially in the case of each sponge an irregular arrange- 
ment of young spicules, which, during growth, becomes a 
regular and symmetrical arrangement, and since the rotation 
of solid structures implies a mechanical cause, and since this 
cause must be uniformly dispositioned with regard to the 
sponge as a whole (since the spicules are regularly distributed 
round the whole circumference of the sponge), there is, on 
this account alone, an 4 priori probability in favour of the 
conclusion that the motion of the surrounding medium so 
functions. In the case of localised secondary forms of the 
triradiates this probability is even stronger since it is incon- 
ceivable that each scleroblastic ‘‘ determinant” in the sponge 
ovum should be guided to its appropriate situation in the 
