STUDIES IN SPICULE FORMATION. 539 
possibly excretory in nature,and have doubtless been described 
under another form. 
Tar DEVELOPMENT OF THE CUCUMARIAN SPICULE. 
As a type of the development of a Cucumarian spicule | 
shall describe that of the commonest form of spicule in C. 
sp.—the (s) of the above classification, —a description 
which will apply to all forms of the large plate spicules of 
Cucumariide with but slight, if any, modifications. 
The spicule first makes its appearance as one or more small 
granules (fig. 3), which rapidly assume the form of a minute 
refringent needle (fig. 4). Despite the statements of Hérouard 
(3) and others, the spicule has not at any stage of its evolu- 
tion the form of “un petit tetraédre.’! The remarkable 
feature about this initial spicular needle is that it can arise 
in connection with either two or four scleroblasts (fig. 4)— 
primary scleroblasts, as we shall call them. The origin of 
the spicule between two scleroblasts is by far the commoner 
mode, but, nevertheless, the needle does sometimes originate 
between four scleroblasts, two on either side of it, and hence 
Cucumarian spicules have two modes of origin. Scleroblasts 
recently divided are fairly frequently met with (fig. 2, a, 6), 
and, since I have never yet seen a spicular needle enclosed 
by a single scleroblast, it is reasonable to assume that the bi- 
division of an ordinary scleroblast is the constant percursor 
to the production of a needle associated with two scleroblasts. 
On the other hand, the only explanation which occurs to me 
of the less usual, but equally certain, tetra-scleroblastic 
origin of the spicular needle is that it is the result of the 
apposition of two scleroblasts (fig. 2, c), which soon after- 
wards divide, thus forming four. The tiny needle thus 
originated elongates to form a rod (figs. 5 and 6), which is 
usually somewhat swollen towards the middle and at the 
extremities. Needles and rods possessing four scleroblasts 
(fairly commonly met with) exhibit no difference in size or 
1 See my paper, ‘On the Development of the Echinopluteus Skeleton.” 
