4 
POPULAR SCIENCE REVIEW. 
when dried, half the gemmule shrinks within the opposite half, 
producing a concavo-convex structure, and that the hilum is 
almost always on the concave side ; indicating that the soft con- 
tents of the gemmule are firmly attached to the hilum, drawing 
it inwards when the former shrink from desiccation. When the 
exterior of the gemmule is examined under a moderate power, 
it appears to consist of an aggregation of stellate discs, closely 
packed (fig. 8), to form a continuous surface. On breaking up 
the gemmule, each disc is seen to be one extremity of a siliceous 
body, like a pair of toothed wheels on an axle (fig. 10). Ehren- 
burg described similar bodies under the generic name of 
Amphidiscus, hence these objects are now known as amphidiscs. 
They are placed with their axles perpendicular to the surface of 
the gemmule (fig. 9), hence their opposite stellate extremities 
form two continuous parallel strata of siliceous matter (fig. 9a-b), 
investing the gemmule. These amphidiscs are imbedded in a 
coat of gelatinous substance, whilst their inner extremities 
rest upon a coriaceous capsule, the surface of which is covered 
with hexagonal areolae (fig. 11). Within this capsule are two 
kinds of cells — an outer nucleated series (fig. 12a), which Carter 
believes to be the instruments constructing the capsule, and an 
inner or central mass (fig. 126), consisting of cells closely cor- 
responding with the ordinary sarcoids, but differing in the 
circumstance that they contain numerous ovules or reproductive 
germs (fig. 13). In due season these cellular contents are 
discharged through the hilum (fig. 12a) into the water, and 
undergo a remarkable development, which has been watched by 
several observers, but most successfully by Mr. Carter, whose 
observations on this point throw an important light upon the 
general structure, as well as the growth of the Spongilla. 
A few days after placing a 'gemmule in water, Mr. Carter 
found that a white flocculent substance had escaped through the 
hilum. Under the microscope the discharged substance appeared 
to have a fiat, transparent, irregular margin, containing numerous 
vesicles ; whilst in its central portion were the ovi-bearing,” or 
reproductive cells. Almost coeval with the discharge of the con- 
tents of the gemmule was the appearance of two kinds of siliceous 
spicula, which are formed in the interior of special nucleated 
cells. One of these is the ordinary double-pointed spicula (fig. 4), 
which appears within an elongated cell (fig. 14). It first presents 
itself as a delicate line, but rapidly grows by external additions, 
until it attains its full dimensions. These additions are com- 
monly made more quickly at one (fig. 15), or occasionally at 
more points (fig. 16) than throughout its entire length; so that 
in its half developed condition it presents one or more bead-like 
inflations (figs. 15-16), which disappear when the growth of the 
spicule is completed. The spicula soon outgrow the cell in 
