OF THE DIAT03IEJE. 59 



individual life, has been justly represented by Mr. Thwaites as an act of 

 gemmation, not of reproduction. 



In the coiu\se of self- division, in some instances at least, a mucous or 

 muco-gelatinous matter is thrown out around the fiiistules engaged. This 

 cii'cumstance did not escape the notice of Nageli ; and Prof. Smith (Si/nopsiSj 

 i. p. 62) has, after noting it in previous pages as'a common phenomenon in the 

 family, thus referred to it in the genus Pleiovsigma : — " While self-division 

 is actively going forward, the mucus generated by the dividing fi^ustules is 

 often so considerable as to produce the appearance and effect of a distinct 

 frond, which assumes the form of a thin pellicle of some little tenacity. At 

 other times, when the mucous secretion does not assume the continuity of a 

 pellicle, it invests the individual frustule with a transparent envelope, which 

 has the appearance of an exterior membrane, and has been sometimes mis- 

 taken for such. On one occasion I also met with the fnistules of P. Hippo- 

 campus enclosed in mucous or gelatinoiLS tubes, precisely like those of a 

 Colletonema ; but these conditions must be regarded, for the present at least, 

 as temporary or accidental, and cannot be admitted into the specific or generic 

 descriptions." 



The process of self- division is affected in some unimportant particulars by 

 the figure and habits of certain genera. Thus in one section of the MelosireWy 

 the fnistules of which have convex ends, Mr. Ealfs points out (A. iV. H. 

 xii. p. 347) that the central line is more strongly marked, and seems to 

 divide the frustule into two equal portions. It becomes broader, and at length 

 double, and ultimately an intermediate growth separates the two halves of 

 the fnistule, which, during this process, do not increase in size ; but when 

 the intermediate space is equal to the diameter of the original frustule, two 

 new frustules are formed, by the addition of two hemispheres on the inner 

 sides of the separated portions. The outer silicious covering still remaining, 

 the fnistules are connected in pairs, and appear like two globules within a 

 joint, as they are characterized by Harvey in Melosira nummuloides, and by 

 Carmichael in M. glohifera. The above description belongs more particularly 

 to M. nmnmulokles ; but the process in the other species of this section is the 

 same : a series of changes, nearly similar, occurs in Isthmia. 



" In this genus," the author quoted says, "■ the mode of growth is very 

 curious. As in most of the Diatomeae, the plant increases by a division of 

 the fnistules ; but in this genus, as also in Biddiilphm and Amphitetras 

 (and in the Achnanthece), two new fi^ustules are formed within the old one, 

 and as they enlarge, ruptui^e it, when it falls off. In these the front portion 

 is at first very narrow, and merely a broad line, but it increases greatly in 

 breadth until the new frustules are fiilly formed." In this description and 

 explanation the widening band or fi'ont portion mentioned is in fact the 

 * connecting membrane ' of Prof. Smith, which, in the genera named, has 

 an extra development, " an extension beyond the sutures of the valves," and 

 also an unusual persistence, retaining the two frustules together after self- 

 division, in such a manner that they seem to be enclosed Avithin an original 

 single frustule, just as Mr. Ealfs describes. 



This longer persistence of the connecting membrane has been noted by 

 Prof. Smith {A. N. H. 1851, p. 4), who writes—'' In some cases, by the 

 new, or rather semi-new frustules proceeding immediately to repeat the 

 process [of self-division], the connecting membrane is thi^own off and disap- 

 pears ; in others it remains for some time, linking the fi'ustules in paii's, as 

 in Melosira and OdonteUa.'" 



Another peculiarity, again, not unfi-equently obtains in this process of self- 

 fission, viz. a departure from the prevailing law of similarity which exists 



