STUDIES IN SPICULE FORMATION. 321 
spicule where branches protrude, apparently caused by them, 
but this seems not always to be the case in the formation of 
unimportant spines.’ I do not know whether the fact that 
the rods of the postero-ventral arms appear at the bends of 
the previously deposited skeleton, cells having previously 
collected there, is of any significance, but it may at least be 
pointed out that such is the case; moreover, in the other 
pluteus which I have carefully examined (that of EH. miliaris), 
the same thing happens, the skeletal rods of the dorsal arms 
arising from sudden bends which here occur in the recurrent 
apophyses. ‘I'he same phenomenon occurs in other plutei, 
though whether secondary branches always arise in such 
positions} I am unable to say. 
That the formation of spicular processes is essential to the 
production of the arms characteristic of plutei is now well 
established. Herbst (8), e.g. showed that arms are not 
developed in plutei reared in sea water deprived of calcium 
salts—a skeleton necessarily being absent under such con- 
ditions. He also showed (9) that if, by immersion in lithium 
water, the calcareous needles are displaced from their normal 
position in the pluteus, arms are produced from portions of the 
ectodermal surface which do not normally share in the for- 
mation of these outgrowths—a result confirmatory of the same 
conclusion. 
As regards the relation between the scleroblast and the 
lime deposited, I can amply confirm the statements of Théel 
as to the deposition occurring in the ‘‘clear homogeneous 
hyaloplasm or ectoplasm”’ of the cells, and not in the vacuo- 
lated endoplasm, and the young triradiate originating in a 
“clear pseudopodial clump” formed by the fusion of the 
ectoplasms of three or more cells. Also, as Théel remarks, it 
is “on account of the transparency of the pseudopodial plasm 
and the opacity of the granular [very much vacuolated in 
E. esculentus] main portion of the cells [that sometimes] 
one gets the impression that the tetrahedron is extra-cellular 
1 In many cases, of course, the secondary arms arise from the angles of 
the already formed skeleton. 
