STUDIES IN SPICULE FORMATION. 291 
edge from those situated more or less deeply in the mesogleeal 
substance. In the former I have observed in nearly every 
instance the presence of a distinct cnidocil, as shown in fig. 
23, but I cannot detect this structure in those more internally 
situated, and it doubtless does not exist in their case. The 
presence of this cnidocil I think distinctly proves the nema- 
tocyst nature of the “ovoid bodies” of Aleyonium, which 
thus, according to Hickson’s account, possesses two kinds of 
nematocysts—one restricted more to the region of the tenta- 
cles, the other to the mass of the colony. 
Thus, in Aleyonium digitatum, in addition to the 
scleroblasts, there exist in the mesoglceal substance, endoderm 
cells, the spherical “ jelly-secreting ” (Bourne) cells, the small 
interstitial cells aud the nematocysts or “ ovoid bodies,” all 
of which latter are more or less distinguishable from the 
former. 
THE ORIGIN AND DEVELOPMENT OF THE SPICULES. 
The scleroblasts, as already stated, are granular, more or 
less spherical cells situated at the periphery under the ecto- 
derm, and probably derived, as Hickson suggests, from the 
interstitial cells of that layer. Further, “ the spicules are far 
more numerous at the periphery than in the deeper parts of 
the colony. This suggests very forcibly that the spicules are 
only formed at the periphery, and that with the growth of 
the mesoglea they become more and more separated from 
one another’’—a suggestion I can amply confirm. The 
spicule first appears in the cytoplasm as a small spherical 
concretion (figs. 1 and 2), and remains approximately spheri- 
cal in its further enlargement until the division into two 
of the nucleus (fig. 3). 
This last statement, I am aware, is contrary to the account 
of early spicule formation given by Bourne, who supplies 
figures representing the young spicule as of very irregular 
shape long before nuclear division occurs. One possible 
