Structure of eell-ivall 



89 



the most delicate striation (e.g. Amphipleura pellucida, various species of 

 Gyrosigma (= Pleurosigma) are frequently used as test objects for the 

 definition and angular aperture of the object-lenses of microscopes. During 

 more recent years our knowledge of the minute structure of the wall of 

 diatoms has been much increased, largely by the investigations of Pfitzer 

 (71), Lauterborn ('96), Schlitt and O. Miiller ('98 '01). 



In the centric diatoms, which are for the most part marine, there is 

 generally a radial disposition of the markings on the valve-faces. In some 



B 



Fig. 62. A C, Isthmia nervosa Kiitz. A, girdle view; B, part of the end of a valve, seen from 

 the inner side, showing the primary and secondary chambers in the wall ; C, ideal section 

 through the primary and secondary chambers of the wall. D, ideal section through the 

 chambered wall of Eupodiscns Argus Ehrenb. p, pores ; po, poroids or dots ; ext., outer 

 side of cell-wall. (A, after W. Smith ; BD, after 0. Miiller.) 



of the larger species the valves exhibit a beautiful areolation, due to 

 numerous, closely adjacent chambers in the siliceous wall. The chambers 

 may be open to the exterior or they may be covered by a thin siliceous 

 membrane, and their inner walls are pierced by exceedingly minute canals 

 which pass right through to the interior of the cell. These minute canals, 

 which are termed " pores," are not present in all diatoms, as some species 



