1857.] THE DTATOMACE/E. 123 



it is more developed at the extremity of the Diatom, forming a 

 kind of pedicel; and sometimes, though very rarely, an exact 

 counterpart of this is produced in the opposite direction. 



The Diatomacece multiply by the separation of the old or pa- 

 rent cell into two new cells. The contents of the parent-cell be- 

 come divided, and a primordial utricle is formed round each por- 

 tion. Shortly these grow into new cells, which either remain in 

 contact, or become separated. In some genera two connected 

 Diatoms are split asunder, and their contents, surrounded by a 

 primordial utricle, form a process called a gonidium. Sometimes 

 numerous rudimentary cells — at first unsymmetrical, afterwards 

 regularly oval or oblong — are developed within the parent-cell, 

 after awhile circulate there, and become discharged by the break- 

 ing up of the old cell. In a sm'prisingly short time they attain 

 or even surpass the size of the parent-cell, which, immediately 

 after discharging its contents, ceases to exist. 



An apparently voluntary motion is displayed by almost all the 

 unattached Diatomacecs, and particularly by those shaped like a 

 boat, as the Naviculm. This motion greatly varies ; sometimes 

 it is calm, sometimes fitful; now progressive, now retrograde. 

 If obstacles interfere, the Diatom diverges from its straight- 

 onward course, and passes alongside the obstruction, or swims 

 round it, its anterior extremity being either raised or depressed : 

 this motion arises from the reception and expulsion of fluids by 

 the Diatom. If these fluids are received at the right side, and 

 expelled at the left, the movement is towards the right, and vice 

 vei'sd. As both halves of the Diatom, physiologically and mor- 

 phologically, are precisely alike, the reception and expulsion of 

 the fluid may take place either to the left or right, and hence 

 the motion may be progressive or retrograde. 



Wherever moisture prevails, one is sm^e to meet with Diatoms. 

 They are most abundant in spring, and are found in puddles and 

 ditches, as well as in the clearest streams. Their production and 

 increase are often astonishingly rapid. On allowing a glass of 

 water to remain for some days in a room, they may be detected, 

 together with minute algse, deposited on the bottom and sides of 

 the vessel. In spring the number of species is comparatively 

 small, and that of the indi\dduals of each species great ; while m 

 summer and autumn the variety of species is much more consi- 

 derable, one gathering often showing from ten to twenty. They 



