318 W. WOODLAND. 
ray is at any point determined by the cells in its immediate 
neighbourhood,! for, if such were the case, the curves of the 
ray would be numerous and sharp, whereas, in actuality, they 
are comparatively few and gradual. It may be added in 
connection with the hypothetical tractive influence of the 
cells on the growth of the initial calcareous granule that the 
undoubted influence which the basal actinoblasts of the tri- 
radiate spicule of Calcarea exert on an adjacent pore-cell or 
cell of the oscular rim (16, 24) supplies a very fair analogy 
for my supposition. 
The above seems to me to be the only possible explanation, 
in our present state of knowledge, as to the cause of the 
triradiate form of the pluteus spicule. Consider the argu- 
ment: triradiate spicules very alike in form, but not exactly 
alike, arise in all plutei in a certain position; the disposition 
of the secreting scleroblasts is in all larve substantially the 
same en masse, but is very different in different larve as 
regards the arrangement of individual cells—from which 
premises the only logical conclusion to draw is that which I 
have stated above. 
Théel suggests an explanation of the triradiate form which, 
though it seems to me wholly untenable, I think is worth 
while quoting, since it contains several very true observations. 
“J could never observe,” he says, “ that the tetrahedron [the 
three-cornered granule] becomes visible before at least three 
calciferous cells have arranged themselves in a heap close to 
the blastoderm. But after this is done the formation of the 
tetrahedron takes place in a clear pseudopodial plasm situated 
between these cells and the ectoderm [see my fig. 15, e. g.] 
and evidently derived not from one cell but from all three 
cells, the pseudopodia of which have united into a small 
clump... Thus, according to my opinion, the calcareous 
tetrahedron is a result of the activity of several cells, which 
1 Judging, however, from the different directions which the three rays of 
the young triradiates in figs. 7 and 16 e.g. have assumed, it would seem that 
adjacent cells have a lot to do with the origin of young triradiates, if not so 
much with their future growth. 
