278 



SURFACE-MARKINGS OF DIATOMACEiE. 



' ' The internal contents of the f rustule never escape at these points 

 when the f rustule is subjected to pressure, but invariably at the 

 suture or at the extremities, where the foramina already described 

 exist. Nor does the valve, when fractured, show any disposition 

 to break at the expansions of the central line, as would necessarily 

 be the case were such points perforations and not nodules.'' And 

 Prof. Bailey has arrived at the same conclusion from watching the 

 results of the action of hydrofluoric acid on the silicified valves, 

 the thinnest parts of which are of course the first to be dissolved, 

 whilst the parts which have been described as apertures are found 

 to be the last to disappear. 



216. The nature of the delicate and regular markings with which 

 probably every Diatomaceous valve is beset, has been of late years 

 a subject of much discussion among Microscopists ; but on certain 

 points there is now a general convergence of opinion. There can be 



Fig. 118. 



Portion of Cell of Isthmia nervosa, highly magnified. 



no question as to the nature of the comparatively coarse areolation 

 seen in the larger forms, such as Isthmia (Fig. 118), Triceralium 

 (Fig. 119), and Biddulphia (Fig. 121) ; in all of which the structure 

 of the valve can be distinctly seen under a low magnifying power 

 and with ordinary light. In each of these instances we see a num- 

 ber of symmetrically disposed areola, rounded, oval, or hexagonal, 

 with intervening boundaries ; and the idea at once suggests itself, 

 that these areolae are portions of the surface either elevated above 

 or depressed below the rest. That the areolte are really depressions, 

 is suggested by the appearances presented by the surface when the 

 light is obliquely directed ; and it may also be inferred from their 

 aspect when viewed by the Black-ground illumination (§ 84), since 

 the areola? are then less bright than their boundaries, less light being 

 stopped by their thinner substance. The view of these objects 



