77 8 
DR. E. B. WILSON ON THE DEVELOPMENT OF RENILLA. 
College, for examination with the polariscope. He kindly undertook to examine 
them, and the following statement of his results is quoted verbatim:— 
“ I find the smaller spicules to polarise perfectly and very brightly, and if the 
spicules are a carbonate, it is probably aragonite rather than calcite, because of the 
bright colour it affords. 
“ Each spicule is made up of a core of crystalline material surrounded by a thin 
layer of non-polarising matter, and this by an outer layer, slightly thicker, and also of 
equal thickness, which polarises as does the core. 
“ The crystalline axes uniformly run parallel to the long and short diagonals, as in 
the sketch, so that the core is a crystal probably of the rhombic system, and the 
outer layer is controlled in its position by this core and has parallel axes with it. 
“ The larger spicules I can best explain by saying that for a moment, on examining 
the slides, I supposed the small ones to be cross sections of the former (larger) ones. 
They are like bones filled with marrow under the microscope, the bone and the 
marrow representing the core and the outer layer of the small ones, and these being 
separated by an amorphous (in one case there were two amorphous layers) layer. In 
this case, the long axis of the spicule is a crystallographic axis. And bone and 
marrow are orientirt alike.” 
From this, the very interesting fact appears that the spicules are formed by a true 
process of crystallisation, though the form and structure of the crystals are modified, 
probably by reason of their deposition in an organic viscous medium. This point, as 
noted below, is one of much theoretical interest. 
In Leptogorgia the characteristic spicules appear in the ectoderm soon after the 
attachment of the larva. They are quite irregularly distributed, and extend up into 
the bases of the tentacles. No entodermic spicules were observed. 
Review. 
My observations on the development of the spicules are in accord with those of 
Kowalevsky on the spicules of Sympodium (Zool. Anzeiger, No. 38, 1879), and 
as Kowalevsky points out, the process is quite similar to the formation of spicules 
in the mesoderm cells of sponges observed by Schultze and Metschnikoff. Their 
mode of development strongly recalls the formation of inorganic crystals in the interior 
of vegetable cells, and possibly indicates the origin of the spicular skeleton. This 
