STUDIES IN SPICULE FORMATION. 537 
may be classified into three kinds: (a) regularly developed 
plates, somewhat thick, which bear prominent knobs on their 
internal face (figs. 17, 18, 19) ; (8) the more common, fairly 
regularly developed plates, which appear to be thinner than 
the knobbed variety, and bear no knobs (figs. 11, 12); 
and (c) very large (three or four times the size of varieties A 
and 8), irregularly developed, generally elongated, thick 
plates with small perforations and no knobs (figs. 25—30). 
These last are usually few in number, and are developed on 
the innermost side of the wall.’ In C. brunnea the larger 
spicules are all of one kind, consisting of more or less irregu- 
lar plates of larger size than usual, fairly thin, with the usnal 
large circular perforations, and with no knobs (figs. 31—36). 
The plate spicules in this species are not so crowded as in C. 
sp., and therefore their development is easier to ascer- 
tain. Unfortunately for me, however, I possess very few 
specimens of this species, and in consequence I have not been 
able to trace the development of its spicules nearly as fully 
as I have in C. sp. 
The scleroblasts or cells which possess the function of 
depositing the spicules are cells which, when free, are more 
or less spherical in contour, stain a light green with lichtgriin 
stain, possess a large spherical nucleus (stained red with 
picro-carmine) with a distinct nucleolus (often with several 
smaller ones as well), and usually also possess a number of 
dark, reddish-brown (so coloured by the lichtgriin) granules, 
which often collect at the cell periphery (figs. 1,2). These 
granules only appear, according to my experience, when the 
cell is stained with lichtgriin ; previously to so staining these 
granules appear as slightly refringent spots in the cell plasm, 
and are almost invisible. Occasionally, but very rarely, these 
granules are absent, and there is apparently no relation what- 
ever between the number and size of these granules and the 
stage of growth of the spicule. Scleroblasts which are not 
free, but which are actively engaged in depositing a spicule, 
also tend to retain the spherical form and appear-as sub- 
spherical masses on the surface of the spicule, the body of 
