STUDIES IN SPICULE FORMATION. 541 
sumably in other Cucumariide, the number and position 
of the scleroblasts adhering to a spicule bear no relation 
whatever to the stage of growth of that spicule '—which fact 
(also illustrated by Alcyonarian spicules) is in striking 
contrast to the conditions found, e.g. in calcareous sponges. 
The scleroblasts appear simply to function as localised stores 
of material needed for the enlargement of the spicule, and 
though, as explained below, it seems probable that the initial 
form of the spicule—the terminally-bifurcated rod—is deter- 
mined by the position and structure of the primary sclero- 
blasts, be they two or four in number, yet beyond this early 
stage of development, their position exerts no influence on 
the manner of growth of the spicule, which, as is well known, 
consists of continuous bifurcations of the extremities result- 
ing in the familiar perforated plates. The body of the 
scleroblast undoubtedly becomes reduced as the mass of the 
spicule increases, though there is no diminution in the 
number or size of the contained black granules, which 
usualiy aggregate in the region of the nucleus. 
To continue the account of spicule development. The 
bifurcations of the initial extremities of the young spicule 
rod elongate and themselves bifurcate (fig. 10), finally uniting 
both terminally and laterally (with respect to the length of 
the spicule rod) to form the first series of four apertures 
(fig. 11). As implied above, the number of scleroblasts 
present at this stage of growth is anything but constant, 
two, three, four, five or more being in different instances 
associated with the spicule, the form or size of which is quite 
independent of the particular number. Bifurcation of the 
spicule extremities continues (fig. 12), and finally results in a 
second series of apertures, and so on. But, in stages of 
spicule growth succeeding the formation of the first series 
of four apertures, it is quite hopeless, at least in C. sp. 
and C. brunnea, to attempt to ascertain the exact number 
of scleroblasts associated with individual spicules. The 
1 Differences of position in the body-wall of different spicules is probably 
largely accountable for this anomaly. 
