NATURAL HISTORY OF THE DIATOMACEjE. 445 



Immediately within the siliceous skeleton of a diatom is supposed by 

 many to be a membrane or skin which bounds and limits the soft parts of 

 the organism ; and it is this membrane, say those who believe in its exist- 

 ence, which secretes or forms the wonderfully sculptured epiderm we 

 have been considering. Some observers think that there is also an out- 

 side membrane, and that it is in it that the silica accumulates. But as 

 many good observers have been able to see neither of these membranes 

 in the living diatom, and as even the believers in their existence acknowl- 

 edge that they are extremely difficult of exhibition, most likely the fact is 

 that the real individual matter of the diatom is a mass of structureless 

 protoplasm, which deposits near its outer portion the silica it has absorbed 

 from the surrounding water. Within the protoplasmic mass is the endo- 

 chromc or colored matter we have already spoken of, and which is most 

 commonly disposed in two portions contiguous to the two valves. In the 

 clear central portion there is often to be seen a little sac or vesicle, which 

 is quite transparent, except at one part, where a minute dot is seen. This 

 vesicle is considered to be the "nucleus" of the diatom, while the dot is 

 the "nucleolus," both of these things being required in a cell constituted 

 under the type established by a German observer named Schwann. But 

 now that Schwann's typical cell, consisting of a "cell wall" containing 

 "cell contents," wherein are found a "nucleus" and often one "nucleolus" 

 or several "nucleoli," is known to exist rarely, we are not surprised if 

 we do not find all of these parts present in a diatom. And now that 

 we understand the internal anotomy of the diatom, without taking into 

 consideration disputed minutiae of structure, we can see how the indi- 

 vidual grows. 



Schwann has shown us what he considers cell growth to be, and it 

 is what is known as "cell subdivision." That is to say, the cell itself is 

 stable as to size, but increase of volume occurs by its dividing into two, 

 and these two into four, and so on, so that if the resulting cells remain 

 united to each other there will eventuate a true increase in bulk, and 

 eventually a large organism like a tree or a man may be formed. But 

 such is not the way that the diatom grows, for it is not a polycellular, but 

 a unicellular organism. There is a large group of very simple plants, 

 even more simple in structure than the diatomacex, for they have no 

 elaborately sculptured siliceous cell-wall, which are known as Pivtophytcs, 



