i897-9 8 -] On the Diatomacece. 355 



top and bottom — to which the term valves is applied. Kunning 

 round the margin of each of these valves is a band or hoop, 

 the one of which slides into the other : these bands or hoops 

 are called the connectives. The four parts — the two valves 

 and the two connectives — thus form a small box, and, if we 

 take a circular specimen as our example, the thing it most 

 resembles is a small pill-box. These valves of the diatom are 

 composed of silex, quite transparent, and practically inde- 

 structible, so that they withstand — except some of the more 

 delicate marine species — boiling in the strongest acids. This 

 boiling process, of which more anon, is necessary to reveal 

 what many consider the great beauty of the diatom valve, its 

 peculiar markings and tracery, the variety of which seems 

 endless. The form of the valve is of all conceivable shapes, 

 from linear to circular. 



The principal of the contents of the diatom frustule are 

 the protoplasm, the endochrome or colouring matter, and oily 

 globules scattered throughout the cell. Embedded in the 

 protoplasm is the nucleus or embryonic germ, the function of 

 which is the multiplication and reproduction of the cell. The 

 whole of the frustule is covered by a mucous membrane, 

 which in some species is prolonged so as to form a stipes or 

 stalk, by which the frustule is attached to some other body. 



I have said the function of the nucleus is the multiplica- 

 tion and reproduction of the cell or frustule. In giving an 

 account of these phenomena, I will follow Dr P. Miguel, who 

 has bestowed much time in their elucidation. The multiplica- 

 tion takes place by the division of the cell, each half forming 

 a new cell. This is effected by the protoplasm increasing, and 

 thus forcing the two valves apart. At a certain stage in this 

 process the nucleus also divides, and thus each half goes with 

 the divided protoplasm : while this is being accomplished, a 

 new silicious covering or valve has been forming upon each 

 of the inner faces, so that when the separation of the two 

 halves actually takes place, each frustule is complete in itself, 

 with its two valves and its connectives, its protoplasm and its 

 embryonic germ, and the other constituents of the contents of 

 the cell. Before the two halves were completely separated, 

 and while they were still being held together by the external 

 mucous membrane, a new silicious valve was being elaborated 

 for each of the halves, so that, when the separation did actu- 



