2 THE STRUCTURE AND LIFE 



Finally, the frustule is coated or enveloped with mucous, or some- 

 times muco-silicious matter, the existence of which can be demonstrated, 

 as Prof. H. L. Smith has shewn, by means of fuchsine or ordinary 

 methylene blue, either in simple solution or with a slight addition of borax 

 (Miquel), which must be applied in very small quantities, so that the 

 water shall be only just tinted. This investment (or coleoderm as it has 

 been called by De Brebisson) is in some cases developed to such an 

 extent that the diatoms, when united together, assume the appearance of 

 higher-branched algae, Ulvacese, &c. Such are the diatoms composing 

 the genera Schizonema, Dickieia, &c. 



Some of the various parts of a diatom require a more careful exami- 

 nation. (0 



The nucleus (fig. i A) is analogous to that of all vegetable cells ; 

 generally a nucleoh can be distinctly seen. In many diatoms the nucleus 



is quasi-invisible, but Ur. Miquel has 

 shewn that it can be made apparent 

 by means of a weak aqueous solution 

 of methylene blue; this colours the 

 chromatine of the nucleus in living 

 diatoms before the peri-nuclear proto- 

 plasm and the protoplasmic mass 

 become coloured in their turn. The 

 best mode of carrying out the opera- 

 tion is to place the living diatoms in a 

 watch-glass, with a little water in it, 



A. Nucleus and nucleole. BB. Protoplasm. j f . f jj f] methylene blue 



B'B'. Membranous cell. CC. Endochrome. anQ tnen t0 aaQ tlie metn y lene DUle 



EE Aj? n i^ bl jl es - FE Valves - solution drop by drop, which should 



G(j, (j Ct . Connecting parts. * J x 



DD. Central cavity (after Mr. J. Deby). be of the strength of one in a thousand. 



The protoplasmic mass (fig. i BB) 

 surrounds the nucleus ; it is finely granulated and attached to the proto- 

 plasm of the cell wall, sometimes by two large bands and at other times 

 by radiating or even anastomosed prolongations of different diameters. 

 The liquid of the cell, enclosing the protoplasm, probably consists of 

 almost pure water. 



The cellular membrane or primordial cell (fig. i B'B') is in contact 

 with the inner surface of the valves. It is enclosed on all sides and of 



e f|5f ,,K 





> nil i 



J- 



G' C 



Fig. i. 

 Ideal Section of a Navicula, 



\ l ) See Mr. Julien Deby's excellent paper : Ce que c'est qu'une Diatome'e in the Bulletins 

 de la Socidte" Beige de Microscopie for the year 1877. 



