298 DESCBIPTION OF {Polygcistrica 



valvular suture are turned towards the observer; the words lateral 

 or side-view, when the general sui-facc of one of the valves is 

 directed to the eye." 



When any two opposite surfaces approximate about the margin of 

 a frastule, the other two sides are reduced to so narrow a band as to be 

 vii'tually obliterated, and are then spoken of as obsolete. 



"Within the lorica is contained the living substance of the frustule, 

 regarded by Ehrenberg as fonning definite animal organs, viz., 

 stomachs, ovaries, and ova, semiaal glands and vesicles. But all such 

 animal organs are ignored by other naturalists; and the contents 

 generally are known as the endochrome, whilst the particular vesicles 

 and granules are represented as similar to the like formations in the 

 Confeiwae, and as constituted of chlorophyll, starch, and mucilage. 

 The number and position of these vesicles vary in individuals of the 

 same species ; sometimes, indeed, specimens occur with no vesicles 

 (stomach-sacs of Ehrenberg) ; and further, a circulation or rotation of 

 the contents of the cell has been observed by I^iigeli in a Gallionella, 

 and by Rev. W. Smith in Surirella. Some of the clear vesicles, 

 Kiitzing has concluded to be oil particles; and affirms, that he has 

 occasionally seen two coalesce, proving the absence of proper walls. 

 The entire endochrome that author also has designated the gonimic 

 substance. 



The prevailing colour of the endochrome of DiatomecB is golden- 

 yellow or brownish ; very rarely gi-ecn, except when the frustules 

 arc ch'ied. 



According to the researches of Niigeli, the Diatomece present, gene- 

 rally, among their other contents, a nuelem, which in some genera is 

 free (isolated) within the general cavity, or parietal — affi-sed to the 

 walls, as happens in GallioneTla. This special organ plays a veiy 

 important part in the phenomena of cell growth, and, in the opinion 

 of Schleiden, precedes and brings about the formation of the cell -wall 

 enclosing it. Niigeli makes two sorts of nuclei, primarij and 

 secondary, attributing to the former the same properties as does 

 Schleiden in the original formation of the cell, and in its subsequent 

 multiplication by self-division. In the latter process, the nucleus is 

 supposed first to double itself, and then the septum to appear, which 

 ultimately halves the frustules. 



