ANIMAL NATURE OF DIATOME^. 511 



plants, and exogenous in animals. He supposes that 

 the quaternary azotised substance acts catalytically on 

 the circumjacent materials to produce organisation. In 

 animals, therefore, the cellular wall acts upon the extra- 

 cellular materials ; whilst in vegetables, the ternary gela- 

 tinous envelope being once formed, the catalytical action 

 of the nitrogenous portion is limited to the interior of 

 the cell itself. We are indebted to Mohl for the discovery 

 of cells that remain permanently limited to this membrane 

 alone, (so called amy lid-cell,) and thus we find an ex- 

 planation consistent with this theory, and also with the 

 facts recently observed in the development of yeast. 



Though it be true that systematic distinctions are 

 always baffled by nature when they are based upon the 

 materiality of characters taken absolutely, it is yet true 

 that general laws admit of no exception, and that a single 

 one would suffice to destroy their generality. The 

 appearance of exceptions proceeds from the complication 

 in which natural facts occur, a complication that frequently 

 disappears, when, instead of characters taken absolutely, 

 we obtain a view of the organic and vital conditions 

 which give rise to them. 



Page 20. The Tabasheer of the Bamboo, which con- 

 sists almost entirely of pure silex, seems to be a product of 

 simple excretion, and therefore in its nature inorganic. 

 But in all other cases in which the presence of silex is 

 manifest in plants, as in the epidermis of Graminea3, 

 Palms, and Equiseta, the part which it takes in the forma- 

 tion of the cell-wall is undeniable. 



The stomatic cells oi Equiseta merit particular attention, 

 both from the silex they contain, and the transverse striae 

 they present on the internal surface. This resemblance 

 to the shield of Diatomese might lead us to believe that 

 we ought to regard it as an argument for maintaining 

 the vegetability of the latter. I do not think that I 

 ought to dwell upon such an objection ; I only notice it 

 because I would not appear to be, or pretend to be, 

 unacquainted with it. Yet it seems to me important in 



