XX INTRODUCTION. 



the shells." (Erom Silliman's Journal for May 1851, quoted 

 in Ann. Nat. Hist. 2nd Ser. vol. viii. p. 157.) 



The usual colour of the living frustide I have stated to be a 

 yellow, with a brown or greenish shade of a greater or less in- 

 tensity ; this coloiu" is found to arise from the contents of the 

 cell, which consist of a mucilaginous fluid, in which float numerous 

 minute granules, generally accumulated in thin layers towards 

 the internal cell-waUs. \Vlien the frustule is so turned that this 

 layer of endochrome is presented edgeways to the eye, the gra- 

 nides appear to be chiefly aggregated into two plates, applied to 

 the opposite sides of the frustule ; and when self-division is in 

 progress, and the cell-contents are divided into two portions, 

 such a separation or temporary aggregation of the endochrome 

 must necessarily ensue; but in the simplest condition of the 

 frustule the contents are difi\ised over the entire surface of 

 the ceU-walls, precisely as may be seen in the cells of many 

 of the larger Algae, or of some water plants of a higher order, 

 as in the leaves of Hydrocharis Morsus-rancB and others. 



The endochrome of the Diatomaceous frustule is not, however, 

 spread with uninterrupted regularity over the entire wall of the 

 cell ; at a spot exactly in the centre of the frustule it is usuaUy 

 absent, being aggregated in a narrow line or ring around 

 a circular space occupied by a transparent vesicle, the nucleus 

 or cytoblast of the cell, which frequently encloses an evident 

 nucleolus. Nor are we to regard the endochrome as being 

 always confined to the surface of the cell-wafl. It is often difi^used 

 throughout the entire cavity, in lines radiating from the nu- 

 cleus. Such a disposition frequently occm's in Pleurosiyma 

 and others, and is invariably found in Navicula eUijJtica. (See 

 Frontispiece, fig. CLII.) 



In most species several smaller globides, in addition to the 

 central vesicle, are found occupying certain fixed positions within 

 the cell; these are probably special secretions from the cell- 

 contents, of an oily nature ; and I am disposed to attribute to 

 them the origin of the peculiar odour exhaled from the frustules 

 in the process of burning. The number of these globules is 

 frequently four, often placed near the extremities, or, more rarely, 

 clustered around the central vesicle. Professor Kiltzing has ex- 



