428 Miscellaneous. 
surface of the body-wall, and three more externally, towards the 
dermal surface. Each of these sets of three cells has a form which 
might be compared to a trefoil, and the whole mass may be described 
as two such trefoils superposed, the cells of one trefoil exactly 
corresponding to those of the other. ' 
The spicule is formed by the three inner cells, a ray being formed 
by each cell. In many instances it appears as if the three rays 
were formed quite separately and afterwards fused at the centre. 
The three outer cells soon lose their rounded form, and, by 
throwing out processes, assume an amoeboid appearance. After the 
spicule-rays have attained a length of 10 or 15 p, the three outer 
cells are no more to be found, having apparently rejoined the flat 
epithelium from whence they came. The three inner cells alone 
secrete the rays, and continue to do so until the spicule is full-grown. 
The spicule-rays soon appear to project beyond their formative 
cells, but are in reality covered by a thin layer of protoplasm. At 
the same time the spicule-sheath makes its appearance as a denser 
layer of substance between the protoplasm of the formative cell 
and the calcareous spicule, and it is by continued calcification of the 
sheath that the spicule grows. 
The spicule-rays attain their full thickness at their bases before 
they have reached their full length. ‘The formative cells remain at 
the bases of the rays until this portion is built up to its full thick- 
ness. ach formative cell then migrates along its ray towards the 
tip, building up the ray to its full thickness as it goes. In the 
fully formed ray the formative cell is found adherent to the extreme 
tip. 
Theoretical considerations. 
(1) The origin of the spicule-forming cells—that is to say, of the 
whole connective-tissue system in these sponges—from the external 
flat epithelium, is another nail in the coffin of the so-called mesoderm 
in these forms. Sponges are to be regarded as two-layered animals, 
composed of a dermal and a gastral layer. The dermal layer is 
differentiated into (1) an external flat contractile epithelium, the 
neuro-muscular system, and (2) an internal connective-tissue layer. 
The gastral layer consists of the collar-cells. The amoeboid wander- 
ing cells are perhaps also to be reckoned with the gastral layer. 
(2) The fact that each ray of a triradiate spicule is formed by a 
single cell shows that each triradiate spicule must be regarded as 
derived from the fusion of three originally separate monactinal 
spicules. This supports Schulze’s theory, namely, that the tri- 
radiates of the more primitive Ascons have arisen as an adaptation 
to the structure of the sponge, and goes against Dreyer’s theory that 
the primitive spicule of all sponges is a tetraxon, a form explained 
by him as the direct mechanical outcome of the vesicular structure 
of living bodies.—Proc. Roy. Soc. vol. lviii. no. 350, pp. 204, 205. 
