x 6 THE STRUCTURE AND LIFE 



matter is as simple as Mr. Deby has made it out to be, sexual reproduction 

 would be very frequently observed, which, however, is not the case. 



Some causes not yet known must therefore occur to retard sexual repro- 

 duction. Otto Muller has tried to investigate what are the causes of this 

 phenomenon being so rare, and for that purpose has attentively studied a 

 filamentous diatom, Melosira arenaria, Moore. As the frustules in this 

 species remain united, it is possible to verify how the decrease in size comes 

 about. ( l ) The following is the result of his researches : 



Considered singly, individual frustules (or individual cells, if preferred) 

 which compose a filament of Melosira arenaria, Moore, have an unequal 

 biological value. Beyond this fact it may be remarked that in many 

 individuals the edge of one or both valves (to the edge of which the connect- 

 ing membrane is attached) is thickened in a peculiar manner ; this thickening 

 is absent in other individuals. The younger valve of each frustule, up to the 

 moment when the division commences, is without any connecting membrane, 

 and is surrounded by the connecting membrane of the older valve. 



The structure of the connecting membrane, differing from that of the 

 membrane of the valve at the lateral limits of the filament, allows a certain 

 microscopical distinction between the, free valve (older and not covered over 

 by a connecting zone), and the younger valve covered by the connecting zone 

 of the older valve of the same cell. The succession, relative position and 

 anatomical structure before mentioned of the free valves and covered valves 

 enables a distinction to be made in the filament between groups of twin and 

 triplet cells arranged in regular order. 



It can be proved in the strictest manner that the cells, the enveloped valves 

 of which have their rims thickened, are produced by their particular ancestors 

 as larger daughter cells ; those, on the other hand, in which this thickness is 

 absent, are produced as smaller daughter cells. 



As soon as the large cells and the smaller daughter cells can be dis- 

 tinguished with certainty, by eliminating the elements of the last formation 

 (of the covered valves), groups of twins and triplets can be traced to their 

 ancestor cells of the (N-i) th or the (N-2) th divisional period, and their 

 special anatomical arrangement, and their relative situation in the filament 

 of each period, can be depicted. 



If in this manner the geneological tree of the filament be reconstructed, it 

 is then found that the smallest daughter cells, which can be easily recognized 

 morphologically by the absence of the thickness in the edge of the younger 



( ) Otto Muller : Die Zellhaut und des Gesetz der Zelltheilungs folge von Melosira arenaria 

 Moore. Berlin, 1883, in 8vo, with 5 plates. 



