: ( 645 ) 
on special places. Seen with reflected light these places are white; 
on the whole those places have a granular or frothy appearance. *) 
As a rule the lamellar structure is very conspicuous. In certain 
cases the carbonised spiculum sheath is likewise visible. It also 
happens that the carbonised and shrunken central thread is seen as 
a flexuous, continuous or broken, black string lying within the central 
canal (fig. 20—21). Of course the best microscopical figures are 
obtained if the spicules are examined in a medium the index of re- 
fraction of which is equal to or comes very near that of the spicopal. 
Controlling experiments sufficiently prove that no artefacts or any- 
thing of that sort come into play through which no conclusion can 
be drawn about the structure of the spicule. The spicopal being in 
slowly by dissolved the method published by Wissman and myself, the 
mieroscope reveals facts wnich are in perfect accordance with those 
obtained in the way described above. One may also combine the 
two methods — heating and dissolving; again the results are the 
same if one follows the process under the microscope. Suppose one 
observes in heated spicules a black central thread with a brownish 
surrounding; suppose the object is mounted in glycerine of about 
the same index of refraction as the spicopal, the external limit is 
clearly visible as a delicate dark line (fig. 22). Some time after the 
action of the hydrofluoric acid the spicule appears as drawn in 
fig. 23. The silica begins to be dissolved as soon as the hydrofluoric 
acid has penetrated the spiculum sheath; the external delicate line 
remains visible but at some distance the limit of the spicopal, now 
thinner, becomes visible. The distance between the sheath and the 
limit of spicopal becomes gradually larger, the brownish surrounding 
of the central thread disappears and finally nothing is left but the 
carbonised central thread and the likewise carbonised sheath (fig. 24). 
This proves, that the brownish colour arround the axial thread does 
not originate from carbonised organic matter. 
1) Biirscutt admits as is well known, that in spicules which are not heated 
jikewise little holes occur and that these holes simply become larger by the process 
of heating and consequently better visible. He says (le p. 248): “Das Auftreten 
der feinwabigen Struktur beruht darauf, dass eine solche auch schon in der nicht 
gegliihten Nadel besteht, jedoch zu fein, um mikroskopisch sichtbar zu sein. Beim 
Gliihen tritt eine Verdampfung des in den Wabenhohlräumchen eingeschlossenen 
Wassers ein und damit eine Erweiterung derselben bis zur Sichtbarkeit. Für diese 
Ansicht spricht vor Allem die Beobachtung, dass wenigstens in einem Fall auch 
eine nicht geglühte Nadel.... den wabigen Bau der Schichten deutlich zeigte.”’ 
Apart from the question whether in unheated spicules a frothy structure really 
occurs or not, it is certain that the dark colour of heated spicules is due to little 
holes, void of air or filled with some gas, say water-vapour. 
