116 Mr. H. J. Carter on the Development 
“horn-cells’’ (‘Annals,’ 1873, vol. 1. p. 6, pl.1. fig. 7, dd). 
Moreover the fibre may present interiorly towards its termina- 
tion a number of conical lines indicative of a succession of 
layers arranged after the manner of a bud (¢b¢d. 1872, vol. x. 
p- 107, pl. vi. figs. 5-7), but added to the surface and not 
produced, as in the vegetable bud, from the axial substance. 
Hence the horny lamine would appear to be deposited on the 
granuliferous axis by the sarcode or parenchyma, although 
by what element of it in particular there is no evidence to 
show. 
So far, then, we may infer that the axial substance is poly- 
morphic and can enclose extraneous bodies, foreign or formed 
by the sponge itself, as the case may be, thus supplying the 
mould or core, and determining, in the first place, the position 
and extent of the kerasine fibre, which is afterwards deposited 
on it by the sarcode or parenchyma to complete the formation 
of the fibre. 
However acceptable this view may be in the main, it should 
be remembered that the axial substance under the microscope 
is very like the “transparent granuliferous sarcode” of the 
sponge generally, and therefore that it may possess the means 
of covering itself with a layer of kerasine in the first instance, 
although the sarcode of the sponge generally may supply the 
subsequent ones, since in many of the Hydroida the horny 
sheath must be formed by the core, for there is no other soft 
substance externally, although where there is a fleshy layer 
externally, as in Hydractinia, the horny structure produced by 
the ‘horn-cells” in the first instance may be subsequently 
thickened by it (‘ Annals,’ 1880, vol. v. p. 455). 
I have stated that the horny lamine of sponge-fibre generally 
do not present a vestige of cell-structure ; and in no instance, 
except Janthella, are they composed of colowred pigmental 
cells ; but I have also noticed in my description of Aplysina 
Jusca (ante) that, when viewed on their edges in a transverse 
section, the horny lamine here do present a faint colowrless 
appearance of cellular structure, especially in the outer layers, 
which seem to lose it and become more homogeneous as they 
become older or mere internal, evidencing, as in the specimen 
of Janthella from Dr. Bowerbank’s collection, that it is formed 
by cells which in the fully-formed lamine are obliterated ; 
while if this be the case generally, then it may be inferred 
that the horny lamine are produced from horn-secreting 
cells in the parenchyma. Where the pigmental cells of Jan- 
thella are empty, as in the instance to which I have just 
alluded, the cellular structure of the fibre is manifest ; but it 
is still, as before stated, tinged of a carmine colour by the 
