DR. E. B. WILSON ON THE DEVELOPMENT OF RENILLA. 
777 
been able to demonstrate it in spicules of one-fourtb the full size. The largest spicule 
figured ( g .) is not more than one-eighth the length of a fully formed spicule. 
The entodermic spicules (figs. 172 3 to 172") make their appearance at a much earlier 
period than those of the ectoderm. They first appear in the lateral portions of the 
peduncular septum as oval, highly refractive bodies, which are plainly visible from the 
exterior (fig. 181). They become in time very numerous and are arranged in an 
irregular longitudinal band on each side of the septum. They appear also in the 
entoderm of the lateral walls of the body and are especially numerous near the 
posterior end. They become easily detached from their points of origin and may often 
be seen suspended in the fluid which circulates in the cavities of the young polyp. 
Most of the spicules of the peduncular septum are formed in its two outer layers, but 
it often happens that a few are developed in the axial cells and when the latter atrophy 
are left embedded in the lamella (see p. 766). 
The entodermic spicules also are formed in the interior of cells which may much 
more readily be demonstrated than the ectodermic spicule-cells. The cells, as shown 
in fig. 172, are variable in form and usually contain distinct nuclei. The calcareous 
matter is first deposited in the form of very minute rounded nodules which, as in the 
case of the ectoderm spicules, may be clearly brought into view by examination with 
polarised light. Examined by ordinary transmitted light they appear in their earliest 
stages as transparent, scarcely visible bodies : or they may be quite invisible. If, 
however, they be examined with the polariser, and the upper prism be rotated, they 
come into view with the greatest clearness ; and by a proper adjustment of the prisms 
both the cell and the calcareous nodules come clearly into view. The spicule-cells may 
contain only one nodule, or two or three may be present, In the latter case each 
nodule appears in some cases to give rise to an independent spicule. In other cases 
spicules may be seen more or less closely united in groups of two, three, or four, and 
it is probable that each such group is developed within a single cell. The form of the 
nodules varies exceedingly, being spherical, oval, or irregularly angular. A not 
uncommon appearance is shown in fig. 172". The spicule has an oval form and its 
substance refracts the light in such a way as to produce two darker lozenge^shaped 
areas at the ends. The fully formed spicules are usually of a smoothly rounded oval 
form but are in many cases obscurely angular at the ends (see figs. 172"). This is 
not definite enough, however, to admit of comparison with inorganic crystals. 
If the spicules be treated with dilute acid, the calcareous matter is dissolved with 
effervescence, leaving a nearly transparent organic basis which accurately retains the 
form of the spicules. In some cases, at least, their organic basis is formed before the 
calcareous matter is deposited. I have sometimes seen cells resembling the spicule=- 
cells, and containing clear bodies quite similar to the calcareous nodules, but destitute 
of calcareous matter. 
In order to determine the molecular structure of the spicules, I submitted a number 
of them to Professor B. K. Emerson, the well-known mineralogist of Amherst 
