354 ANIMAL NATURK OF DIATOMEjE. 



parative. But the greater peril lies in the very nature of 

 the subject ; because two bodies, very different in degree 

 of organic simplicity, may appear very similar to each 

 other in our eyes, though armed with great magnifying 

 powers. And since we are now strictly endeavouring to 

 discover the characters of an element, which in respect 

 to us is more simple than any other, it is to be feared 

 that in some cases we may take some more complex 

 thing for a cell. 



Every organic being, if observed at the first instant of 

 its appearance, presents itself as a simple cell. When- 

 ever we have a cell before our eyes, this cell is either the 

 rudiment of an ulterior organism, or it is capable of 

 existing by itself and remaining permanently in the same 

 condition, or it is an elementary part of some organic 

 tissue from which it has been separated. Everything 

 anterior to the appearance of the cell, considered in its 

 general condition, may, in the actual state of our know- 

 ledge, be considered as still uncertain, or not fully 

 demonstrated; and therefore it is that we take our 

 starting point from the already beautiful and well-formed 

 cell, because it is always identical in respect to its prin- 

 cipal characters. Its external coating solid, quite con- 

 tinuous, transparent, without any indication of particular 

 structure, viz., without a heterogeneous disposition of 

 molecules. The contained substance liquid, solid or 

 gaseous, different from what remains outside the cell. 

 Nucleus adhering to the inner surface, or detached within 

 the cavity. Nucleolus within the nucleus, distinct by a 

 different refraction of light or by colour, or by a different 

 appearance under chemical reagents. Another general 

 fact, common to all living cells, is an incessant mutual 

 interchange of materials between the fluid within and that 

 external to the cell, through the solid tegument of the 

 cell itself causing perpetual variations in the quantity 

 and quality of the former. 



So far the conditions are common to the two or- 

 ganic kingdoms. We will now compare together such 



