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starker wirki, als der At'liseiikaiial diircli Aii(](»siuig erweitert wird, 

 indeni daiiii eiiie grössere Meiige der Siiure ziir Verfiigiiiig slelit." 

 We believe, oji the contrary, that it really follows from the obser- 

 vation that the inner parts of the spicopal are more easily dissolved 

 than the outer ones. For we see the sharp conical funnel long 

 before any trace of reaction is to be seen on the rest of the fracture. 

 The borders remain intact for a considerable time. And we get the 

 same view by boiling in a solution of caustic potash. Only when 

 the conical hole has attained a certain depth, the dissolution of the 

 spiculum from the exterior commences. Finally there remains a 

 kind of tube, formed of silica in which the axial thread is laying 

 isolated, until all the spicopal has entirely passed into dissolution. 



If we study the way in which the hydrofluoric acid acts on com- 

 pletely intact spicules, some difference may be seen, according as we 

 have to do with sharply pointed needles or with those of which the 

 apex is rounded off. Blunt styli, i. e. transitions to strongyli, resist 

 the acid for some time ; but once the dissolution has begun from 

 the exterior, the process proceeds regularly, the spiculum becomes 

 thinner and thinner. 



It seems that in pointed styli, the apices are first attacked ; in 

 such cases we see the axial thread gradually coming free by external 

 dissolution of the spicopal. Sometimes it may be observed that, in 

 addition to the dissolving process as described above, a hollowing 

 out along the axial thread takes place. In other cases, however, 

 this is not seen, and we get half dissolved spicules in which the 

 axial thread is partly freed, partly enclosed in a coat of spicopal, 

 thus strongly resembling whips ; the more so as the thread is gene- 

 rally flexible, whereas the rest is still straight and rigid. 



BüTSCHLi has already remarked that there are sometimes seen "durcli 

 lokale starkere Auflösung der Kieselsubstanz zellenartige Vertiefungen 

 der Nadeloberflache". "Indem diese Vertiefungen schliesslich zu Löchern 

 werden, die bis zum Achsenkanal reichen, wird dieser der Flusssaure 

 zuganglich und nun beginjit von diesen Löchern des Kaiials aus .... 

 die innere Auflösung der Kieselsubstanz unter Entwicklung zweier 

 trichterförmiger H()hlen . . . ." This observation we can confirm ; we 

 consider it another proof that the spicopal in the neighbourhood of 

 the central thread is more easily dissolved than the peripheric mass ; 

 the observation can hardly be explained in another way. 



Spicules, dissolved in the described way, show, that there remained 

 not only after the dissolution of the spicopal an organic central thread, 

 but also a very thin coat, which covered the exterior layer of the 

 spicopal. This coat, which represents the true spicule sheath, is 



2-* 



