284 BINARY SUBDIVISION OF DIATOMACEjE. 



shown at Fig. 121, a, c, just as a new cellulose-wall is generated 

 in the subdivision of other cells ; and this valve is usually the exact 

 counterpart of the one to which it is opposed, and forms with it a 

 complete cell, so that the original frustule is replaced by two 

 frustules. Sometimes, however, the new valves seem to be a little 

 larger than their predecessors ; so that, in the filamentous species, 

 there may be an increase sufficient to occasion a gradual widening 

 of the filament, although not perceptible when two contiguous 

 frustules are compared ; whilst, in the free forms, frustules of 

 different sizes may be met-with, of which the larger are more 

 numerous than the smaller, the increase in number having taken 

 place in geometrical progression, whilst that of size was uniform. 

 It is not always clear what becomes of the 'hoop.' In Melosira 

 (Figs. 131, 13*2), and perhaps in the filamentous species generally, 

 the ' hoops ' appear to keep the new frustules united together for 

 some time. This is at first the case also in Biddulphia and 

 Isthmia (Fig. 134), in which the continued connection of the two 

 frustules by its means gives rise to an appearance of two complete 

 frustules having been developed within the original (Fig. 121, A, c) ; 

 subsequently, however, the two new frustules slip out of the hoop, 

 which then becomes completely detached ; and the same thing 

 happens with many other Diatoms, so that the 'hoops' are to be 

 found in large numbers in the settlings of water in which these 

 plants have long been growing. But in some other cases all trace 

 of the hoop is lost ; so that it may be questioned whether it has 

 ever been properly silicified, and whether it does not become fused 

 (as it were) into the gelatinous envelope. — During the healthy life 

 of the Diatom, the process of self -division is continually being re- 

 peated ; and a very rapid multiplication of frustules thus takes 

 place, all of which (as in the cases already cited §§ 202, 210) must 

 be considered to be repetitions of one and the same individual 

 form. Hence it may happen that myriads of frustules may be 

 found in one locality, uniformly distinguished by some peculiarity 

 of form, size, or marking ; which may yet have had the same remote 

 origin as another collection of frustules found in some different 

 locality, and alike distinguished by some peculiarity of its own. 

 For there is strong reason to believe that such differences spring-up 

 among the progeny of any true generative act (§ 219) ; and that 

 when that progeny is dispersed by currents into different localities, 

 each will continue to multiply its own special type so long as the 

 process of self-division goes on. 



218. It is uncertain whether the Diatomaceae also multiply by 

 the breaking-up of their endochrome into Gronidia, and by the 

 liberation of these, either in the active condition of 'zoospores,' or 

 in the state of ' still ' or ' resting ' spores. Certain observations by 

 Focke,* however, taken in connection with the analogy of other 



* ••Physiologisch. Studien," Heft ii. 1853. 



