280 THE MICROSCOPE AND ITS REVELATIONS. 



finally a complete double partition, as in other instances ( 252). From 

 each of its adjacent surfaces a new siliceous valve is formed, as shown at 

 Fig. 167, 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, each of which has one old 

 and one new valve, just as in Desmidiaceae ( 264). Generally speaking, 

 the new valves are a little smaller than their predecessors; so that after 

 repeated subdivisions (as in chains of Isthmia), a diminution of diameter 

 becomes obvious. But sometimes the new valves are 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. 177, 178), 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. 181), in 

 which the continued connection of the two frustules by its means give 

 rise to an appearance of two complete frustules having been developed 

 within the original (Fig. 167, A, c); subsequently, however, the two new 

 frustules slip out of the hoop, which then becomes completely detached. 

 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 prop- 

 erly 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 repeated; and a very rapid multipli- 

 cation of frustules thus takes places, all of which (as in the cases already 

 cited, 229, 271), 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 ia 

 strong reason to believe that such differences spring-up among the pro- 

 geny of any true generative act ( 229); and that when that progeny is. 

 dispersed by currents into different localities, each will continue to mul- 

 tiply its own special type so long as the process of self -division goes on. 



279. It is uncertain whether the Diatomaceaa also multiply by the 

 breaking-up of their endochrome into segments, 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, 1 however, 

 taken in connection with the analogy of other Protophytes, and with the 

 fact that the zygospore-frustules almost certainly thus multiply by gonidia 

 ( 280), seem to justify the conclusion that such a method of multiplication 

 does obtain in this group. And it is not at all unlikely that very considerable 

 differences in the size, form, and markings of the frustules, such as many 

 consider sufficient to establish a diversity of species, have their origin in 



1 "Physiologisch. Studien," Heftii., 1853. 



