DIATOMS. 257 



the two views. In Pinnularia it is easy to free the one half of 

 the cell-wall from the other by pressure or by chemical reagents, 

 and, moreover, here and there dead specimens are found in which 

 this process has more or less completely taken place. With 

 pressure the girdles easily break at some little distance from 

 their edge, and along a line parallel with it. These lines, one near 

 each edge, and therefore two in girdle- view, are often recognisable, 

 and may be thin parts of the girdle. They do not extend to the 

 ends of the cell. 



Cell-contents. The contents of the cell present a somewhat 

 different appearance according to whether we have a valve- or 

 girdle-view. In the former (Fig. 97, A), a median clear strip 

 traverses the cell from end to end ; the colourless cytoplasm of 

 the cell is therefore visible. In the mid-length of the cell it 

 appears collected into a bi-concave protoplasmic bridge. In this 

 " bridge " lies the nucleus, not always readily visible without the 

 use of reagents, and with a comparatively large nucleolus, Bound- 

 ing both sides of this clear band, with a tolerably smooth or 

 undulating outline, are the brown-coloured chromatophores, the 

 endochrome plates. These lie, therefore, on the girdle-sides. 

 In the protoplasmic " bridge " can be seen narrow rodlets, con- 

 nected in pairs, the significance of which is unknown. Lastly, in 

 the cell-sap lie usually, but not always, larger and smaller oil- 

 drops, In the girdle-view the cell-body appears uniformly brown, 

 because here the chromatophore covers the whole colourless 

 lining protoplasmic layer. Only at the two extreme ends of the 

 cell does the colourless protoplasm come to view. The chromato- 

 phore is uniformly dense and uniformly coloured, without visible 

 differentiation. In girdle-view also the central collection of pro- 

 toplasm appears to have the form of a bi-concave bridge. 



If we now look over our former preparation of Cladophora, we 

 are pretty certain to find diatoms clinging to this alga. They 

 were fixed and stained at the same time with the alga, and we 

 shall see the stained nucleus beautifully in each cell. 



Amongst a large number of examples of Pinnularia we may 

 here and there find one double. These are sister-cells, which 

 have recently resulted from the division of a mother-cell. They 

 cling to one another with their valve-sides, and, if the wall is fully 

 developed, we can determine that the girdles of the two inner 



17 



