STUDIES IN SPICULE FORMATION. 345 
shape of a spicule. I have first discovered calcareous matter 
inside them when they have attained a length of about 
0:20 mm.” All who have investigated the development of 
calcareous spicules know the extreme susceptibility of these 
structures to traces of acid, and there can be no doubt but 
that Théel’s empty sheaths simply represented the proto- 
plasmic investment of dissolved spicules. It is true that the 
“sheath”’ produces the spicule, but it is no less true that the 
form which the deposited lime assumes determines the shape 
of the ‘‘ sheath ’—the two grow pari passu. 
The same criticism probably also applies to Chun’s wonder- 
ful account (1) of the development of the calcareous wheels 
of the Auricularia larva of Holothurians published ten years 
later. As Théel remarks concerning this account, “there 
seems to be scarcely anything similar observed in other 
Echinoderms.” Chun states that when the scleroblast has 
attained a size of 0:03 mm., “there appears within the old 
cell-membrane a new one, which has an undulating outline 
towards the circular margin and speedily assumes a star- 
shaped form,” and ultimately ‘‘the rays become united by a 
peripheral membranous ring. Itis nowimpossible to mistake 
the mould of the subsequent calcareous wheel, prepared as it 
is by the complex folds of an internal membrane... . 
Moreover, the calx is actually secreted into this organic 
matrix formed by the skeletogenous cells as into a mould, 
and in such a way that (as the older accounts already teach 
us) calcification takes place first in the nave, then in the 
spokes, and finally in the felly of the wheel’’—the nuclei 
present migrating from the centre to the periphery in corre- 
spondence with the extension of calcification. Before accept- 
ing this account of this “unique” mode of origin of the 
Auricularia wheel, I should have to be thoroughly convinced 
as before that the presence of empty sheaths or moulds was 
not due to traces of acid contained in the reagents employed 
in the preservation of the wheels. As I know from ex- 
perience, the calcareous portion of the skeleton of the echino- 
pluteus larva, despite the utmost precautions taken to ensure 
