146 W. WOODLAND. 
graphy concerning the scleroblastic development of tetra- 
xonid and monaxonid spicules, we may say without hesita- 
tion that the vast majority of the spicules, widely as they 
may differ among themselves with respect to form and size, 
arise each as a granule contained within a single scleroblast. 
The few siliceous spicules which do not thus arise from a 
single centre, but from several granules contained within the 
scleroblast (e. g. orthodragmata, Carter’s bundles of ‘‘ tricur- 
vates,” etc.) are exceptional, and in every one of these 
cases the “spicule” consists of several parts throughout its 
existence. J am not aware of a single instance in which 
the body of a siliceous spicule-individual is built up by the 
fusion of originally separate parts, i. e. visible parts, though 
statements have recently been made to the contrary (see 
below). 
Further, with the exception of those spicules which attain 
a relatively large size (e.g. the monaxons of Esperella, 
Ketyon, and ethya, the large tetracts of Geodia and the 
spherasters of ’ethya) all these spicules throughout their 
existence as skeletal elements of the sponge-body remain 
enveloped by the distended substance of the single sclero- 
blast in which they arose; in other words, the nucleus of the 
scleroblast does not divide (figs. 1O—18). 
It is needless for me to enter into details in connection 
with the morphogenesis of each type of spicule; it suffices to 
say that careful work has proved that in the vast majority of 
cases each spicule, whatever its ultimate shape, originates as 
a granule, and only by degrees assumes its final form whilst 
enveloped by the formative scleroplasm. Also, so far as we 
know, all growth of the spicule is accretionary and never 
interstitial, and this fact is constantly to be borne in mind 
when the identification of young forms of certain types of 
spicule is in question. ; 
As stated above, the building up of siliceous sponge 
spicule-individuals by the secondary fusion of separate de- 
I have studied, has the scleroplasm associated with the spicules not been 
hitherto described (see figs. 10—17).. 
