STUDIES IN SPICULE FORMATION. ole 
for the assertion that the tetrahedron is the primordial form 
of calcareous deposits in Echinoderms, does not figure any 
such stage in the development of the wheel-and-anchor 
spicules of Synapta inhaerens, and, indeed, admits that 
such does not here occur. Fewkes (5) does not describe such 
in Ophiopholis aculeata, nor does Gerould (7) in Caudina 
arenata, nor Garman and Colton (6) in Arbacia punc- 
tulata, and many other cases might be cited. Minchin (16) 
is equally positive on this point for Ascon sponges, Chun (2) 
for Auricularia larve, and von Koch (12) for the Alcyonarian 
genus Clavularia. On the other hand, Semon (19) des- 
cribes most distinctly the young tetrahedron spicule for the 
holothurian Chirodota venusta, also Herouard (11) for 
certain other holothurians, and Théel (21) in his excellent 
paper on the development of Echinocyamus pusillus. 
Now all of these last three authors admit that calcareous 
spicules are deposits of mesenchymatous cells, and, indeed, 
Herouard and Théel endeavour to trace the form of the later 
deposits to the disposition of the secreting cells, and yet, this 
being the case, it is deemed necessary to have initially a 
visible crystalline structure as the basis upon which the future 
spicule is to be developed! Probably having in mind the 
“integrant molecule” or crystal particle of crystallography, 
the young stage of the triradiate spicule, which, from a 
crystallographer’s point of view, might at first sight be mis- 
taken for a tetrahedron (see text-fig. 7 below), is deemed to 
be such. Now this assumption is a very doubtful one for two 
reasons: (1) because Semon (with Fewkes, Selenka, Ludwig 
[14] and others) admits that the calcareous deposit originates 
in the interior of a cell as an approximately spherical grain, 
and that this grain forms the nucleus of the future spicule: 
hence the tetrahedron can at most only be the first “ definite ”’ 
form to be assumed by this grain,! from which it is to be 
1 Théel (21) disputes this in the following passage :—‘‘ According to 
Selenka and Semon the tetrahedron not only has originated in a single cell, 
but also arises directly from the calcareous granule of uncertain shape which 
is present in it, and which consequently should form the centre in the future 
