STUDIES IN SPICULE FORMATION. 513 
scleroblasts are present), it is probable that in Thyone, as in 
Cucumaria, these spicules have two modes of origin—the bi- 
and tetra-scleroblastic—the former being by far the more 
common. Owing, however, to my specimens of Thyone not 
being very young, I have not been able to find in this genus 
the youngest stages of these two modes. 
The further development of these Thyone spicules is not 
precisely the same in every case, as will be seen by comparing 
figs. 50 and 51. Also, unlike the Cucumarian spicules, the 
perforations succeeding the first four formed are much smaller, 
attaining a minimum at the extreme periphery of the nearly 
circular plate. As before remarked, the vast majority (at 
least 70 per cent.) of the adult forms of these spicules 
possess only two scleroblasts (figs. 49—52 and 56), and so 
again differ from the spicules of Cucumaria. A few (an 
appreciable percentage) possess four scleroblasts (probably 
tetra-scleroblastic in origin), and also a few undoubtedly 
possess three scleroblasts, so that division of the scleroblasts 
evidently occurs. The reason why it is difficult to suppose 
that all spicules with four scleroblasts attached are derived 
from spicules possessing only two scleroblasts is that four 
scleroblasts are so often (relatively) found associated with 
young stages of these spicules (I believe I have seen a 
terminally-bifurcated rod possessing four scleroblasts, but it 
was not sufficiently certain! to figure). I must further 
mention that I have occasionally come across well-developed 
spicules with only one scleroblast adhering (once or twice 
with none at all), but, of course, in these few cases it may 
safely be assumed that one scleroblast has become detached 
during preparation of the slide, since, apart from this, there 
is no evidence whatever for supposing that these large 
spicules of the (6) variety can originate in one cell. 
As regards the position of the nuclei on the spicule, in 
almost every case the two (or four) nuclei are situated at the 
1 As in Cucumaria the observer has to distinguish between scleroblast 
nuclei and the nuclei of adjacent fibres, but in Thyone this is not so difficult. 
“Green” cells are also present, but they are scarce, 
