Navtculacea,] infusorial animalcules. 299 



Kdgcli further describes the cell formation in BiatomacecB, as 

 taking place around the whole contents of the parent cell: that 

 " diii-ing the production of the septum, the secretion of gelatinous 

 matter continues over the whole surface of the cell contents. The 

 production of the septum itself may be thus explained : the contents 

 separate into two parts, each becoming invested with a membrane, 

 which appears as a thin wall between them, but is actually com- 

 posed of two lamellae." 



Oiu' space, and the nature of the treatise, forbid entering more 

 largely on this interesting but much vexed question of the nuclear 

 development of cells. Those readers who desire to follow up the 

 subject, cannot do better than study the learned disquisition by 

 Niigeli on cell-formation, quoted in the preceding pages. 



The Rev. W. Smith has given the following succinct account of 

 the process of fissiparous, or self-division: "At first, the siliceous 

 valves (as seen in front view) are in close contact at their suture 

 (P. 18, f. 20), but their adherence is speedily disturbed by the 

 dividing process which these minute organisms are constantly under- 

 going. The first step in this process is the gradual separation of the 

 valves, an efiect apparently produced by the expansion of the internal 

 membrane. Pari passu with the retrocession of the valves, the cell 

 wall exposed between their edges is being covered with a deposit of 

 silex, and the frustule now consists of two symmetrical valves, 

 united by a plate of silex (P. 18, f. 23) which either forms a con- 

 tinuous ring (P. 18, f. 22) or consists of two portions united at the 

 extremities of the valves. This plate, with the underlying cell- wall, 

 may, for the sake of distinctness and future reference, be termed the 

 connecting membrane. 



" When the connecting-membrane has been fonned of sufficient 

 width, the original cell, i)robably by the doubling in of its wall, 

 becomes divided into two, and immediately secretes, at the line of 

 division, two new siliceous valves, symmetrical with, and closely 

 applied by their edges to the original halves, and thus the self- 

 division is complete, and two perfect frustulcs have been the result, 

 (P. 18, f. 20.) 



" In some cases, by the new or rather scrni-new frustulas imme- 

 diately proceeding to repeat the process, the connecting-membrane is 

 X 2 



