From the North Sea. 83 
needle. Figs. 15 and 16 show beginnings of such branches 
directed towards the middle of the needle; figs. 17 and 18 the 
same directed towards the point. Sometimes the branching is 
double crosswise, four branches with four canals (fig. 19), 
sometimes regularly, sometimes rather irregularly, or in con- 
nexion with bifurcation (fig. 20). I have, besides, several 
times found an irregular heap of round, bladder-like tuberosi- 
ties (figs. 21, 22, 23), to which the central canal gives no 
branches. Often there are spicules with graduated points 
(figs. 24, 25) ; very seldom their surface is studded with short, 
pointed projections (fig. 26). 
When the spicule is perfectly entire and uninjured, the con- 
tents of the central canal, even after boiling in nitric acid, re- 
tain their transparency ; but if the spicule has been broken, 
even scarcely perceptibly, at the outermost point, the canal is 
partly filled with long, mterrupted columns of gas, less trans- 
parent than the lumen of the canal (figs. 28, 29, 30). 
Prof. Lieberkiihn observed the first formation of siliceous 
spicules in young individuals of Spongilla*. In a cell with 
nucleus and nucleolus there appears among the granules a little 
ball of silica, from which, in opposite directions, but not exactly 
in the same straight line, shoot out two points, which are little 
by little elongated, until they form spindle-shaped needles, the 
ball remaining near the middle as the nodule. It is hardly to 
be doubted that the inflation or nodule in the spindle-shaped 
needles of our sponge, and which, as long as it is of small 
size, receives no branches from the central canal, is the part 
earliest formed—the siliceous ball. Of the growth of the 
needle, free in the parenchyma, we know at present very little. 
It increases by layers one over another. Prof. Kolliker, who 
regards the canal as a solid fibre of soft organic matter, on 
which, within the cell and from its contents, silica is deposited, 
supposes that the spicule increases by secretion of silica from 
the parenchyma in layers one above anotherf. In our sponge 
these layers are scarcely discernible. But another siliceous 
sponge from the Arctic Sea has offered some observations 
which may deserve to be previously mentioned here. The 
layers are very distinct, and seem to be alternately soft and 
hard. A spicule has lost, near the end, its exterior layer, so 
that the point projects beyond the remaining part of it, as out 
of a sheath. Between the outermost broken lamella and the 
exterior surface of the uninjured point there is a space, the 
former contents of which, a soft substance, have disappeared, 
the Canada balsam now occupying their place. If one of the 
* Miiller’s Archiv, 1856, p. 408, t. 15. f. 17-25. 
+ Icones histiologicee, i. p. 61. 
6* 
