STUDIES IN SPICULE FORMATION. 295 
be derived from the original “corner” or end, but from a 
process developed to one or the other side of it. In some 
cases also the rounded extremity of the simple dumb-bell 
divides into (i.e. the rim develops into) many more than 
three or four regions, and the resulting spicule is then more 
complicated at its extremities, but this is rare in A. digi- 
tatum, 
In every case I failed to observe any relationship between 
the position of the nuclei and the development of certain 
processes, and it seems certain that after the first elongation 
of the spicule the nuclei play no further part in determining 
its form. 
REMARKS ON SOME OTHER FRATURES OF THE SPICULES. 
On observing a section of Aleyonium digitatum con- 
taining spicules, preferably under a low power of the micro- 
scope (say x 500 diameters), one cannot fail to notice that 
each of the larger spicules is contained in a distinct cavity 
apparently formed in the substance of the mesoglcea by, and 
during the growth of, the spicule itself, since the thickened 
edyves of the mesogloeal substance constituting the walls of 
this cavity are distinctly supported by the spicular processes 
—like tent canvas on a pole (see figs. 16 and 17),—and hence 
must have been displaced by their formation. This simple 
fact} serves to throw some light upon the physical consistency 
of the mesoglceal substance, and possibly has a bearing on the 
problem as to the causes of the various forms which spicules 
assume (see further). 
The figures of the spicules provided in the plates illustrate 
well another fact of importance, and that is the large amount 
of organic matter contained in the substance of the spicules. 
Fig. 6 shows in optical section the concentric structure (pro- 
bably indicating periodic growth) of the spicule axis, which 
stains pink with picro-carmine, and it is owing to the presence 
of this axis that all the spicules appear pink in colour. The 
1 Possibly an artefact. 
voL. 49, PART 2.—NEW SERIES. 21 
