246 W. WOODLAND. 
Each of the three separate primary needles being thus 
deposited in connection with a pair of the sextet cells situated 
approximately on a radial line, and these three needles 
collectively having a triradiate disposition corresponding to 
that of the original trefoil, the next step in the growth of the 
compound spicule is the junction of the three needles cen- 
trally and their individual thickening (figs. 19 and 83). 
About this time also the two cells connected with each of the 
three rays become more easily distinguishable into (1) a 
basal cell, which is outer in position, i.e. nearer the dermal 
surface, and remains until a late stage of development at the 
base of the ray, where it is wholly active, with its two 
companions, in firmly cementing, at their central junction, 
the proximal portions of the three constituent rays of the 
spicule, and (2) an apical, or inner cell, which advances in 
the direction of the ray at its extremity, and is, indeed, 
chiefly concerned in its construction. As the figures show, 
the apical cell (more deeply coloured for the purpose) lies 
well this side of the spicule ray, and therefore next the 
gastral epithelium, whereas the basal cell is situated pretty 
well in the plane of the spicule and in one of the interspaces 
contained by the rays. In figs. 19¢ and 19d, and some 
others, are shown spicules which are very hollow in appear- 
ance, but this feature is, as I subsequently ascertained from 
Prof. Minchin, due to corrosion by traces of acid contained 
in the glycerine in which the spicule preparations were 
mounted, and is therefore purely artificial in origin.! The 
three apical cells, having built up the three rays of the 
spicule to their full length, leave the spicule for the surround- 
ing mesogloea; also, about the same time, the basal cells, 
having effected the junction of these three rays at their 
1 The spicules of S. coronata are much more easily attacked by acid than 
those of S. ciliata, though so similar in appearance, and Prof. Minchin 
informs me that the same is the case with Leucosolenia variabilis and 
L. complicata—the spicules of the former species being much more sus- 
ceptible. Great care must be exercised to ensure that all reagents used in 
connection with the preparation of calcareous sponge spicules are neutral, 
