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aceolés...” Of course Topsent has not overlooked that spicula have 
a different appearance whether seen “de profil” or ‘‘de face”; but 
evidently he did not pay attention to intermediate positions such as 
can be seen if one allows them to turn over. I drew attention 
(1902 p. 170) to the fact that in almost all cases the twisted character 
becomes plain enough by applying the above device. 
However, there are some more methods to make out the shape 
and the structure of spicules (Cf. Vosmaer & WiJsMaN, 1905 p. 745). 
One of these methods is heating. In using this method it is, 
however, not indifferent in what way it is applied. If isolated 
spicules (e. g. styli of Zethya) simply dried in the air, are heated 
on a platina spatula immediately above the flame, a brownish colour 
soon becomes visible. If they are further heated the brownish tinge 
turns into white. It then frequently occurs that a crackling noise is 
heard and that spicules or portions of spicules jump off from the 
spatula. Such spicules, seen under the microscope, generally appear 
to be cracked or broken; they are brown or black, some were 
quite misshapen as if the spicopal had been partly melted. 
How can these phenoma be explained? BowERBANK ascribed the 
brown or black colour to carbonised organic matter, but KOLLIKER 
proved that the colour can certainly not wholly be explained in this 
way. Indeed, in some parts the colour is brown only in transmitted 
light, whereas it is white in reflected light; consequently KOLLIKER 
declared those parts to contain microscopical air-bubbles. Quite correctly 
„Börscnrr (1901 p. 240) remarks that KöLLiKER where he speaks of 
“Luft”, in fact means “Gas”. Wijsman and myself have demonsirated 
(1905 p. 28), that spicopal is a form of hydrated siliceous acid, 
which can give off water in an atmosphere dried by P,O,. It is, 
therefore, very likely that when the spicules are treated as described 
above, a portion of the water becomes water-vapour. The tension of the 
heated globules of steam of course can be great enough to make 
the spicule explode. This explains at the same time the crackling 
noise and the jumping off from the spatula. Still, it need not come 
to this; hence we see some spicules only slightly cracked, not broken 
or deformed. 
If, however, spicules are not simply dried in the air, but, by 
slowly warming on asbestus for several days or by P,O,, water is 
taken from them, and they are afterwards very carefully heated, 
then it is possible to prevent any cracking. Spicules treated in this 
way show quite other details. First of all the carbonised central 
thread is clearly visible. In some spicules the rest of the spiculum 
remained quite transparent; in others a brownish colour is to be seen 
