OF THE DIATOME.E. 67 



" In Cosmarium margaritifermn and Staurastrum dejectum, it may be easily 

 obseiTed that a slight elongation of the isthmus, and the fonnation of a septum 

 passing across the middle of this, precede the aj^pearanee of new half-cells 

 in the deep constriction. It is after the appearance of the septum that the 

 old waU of the mother- cell breaks by an annular fissure exactly at the place 

 where that septum is formed. The two halves of the old cell-coat are then 

 separated by the bulging- out of the younger, inner layers of membrane, not 

 fii^mly adherent to the old portions. The new halves are at first lined only 

 by protnided portions of the peUicle of theii' contents (outermost layer of 

 the parietal coats of protoplasm) belonging to the older half-ceUs ; from the 

 moment only of the dehiscence of the old cell- coat, does a portion of the 

 granular contents of the older cell-halves make its way into the new emerging 

 halves. 



" In like manner, doubtless, occurs the cell- division of Micrasterias, of the 

 large foiTas of Euastrum, Cosmarium, Staurastrum, and other Desmidieae, 

 only that they have not been observed completely, because these larger Des- 

 midieae very seldom multiply by division out of their natural stations. The 

 ceU-di\dsion of the Diatomeae that have hitherto been observed in vegetative 

 multipKcation, differs in essential points from that just described. 



'' When a cell of Navicula {Pimiularia) viridis is about to divide, there 

 appears upon one of the secondary sides (front view of English authors), 

 parallel to the primaiy sides (the furrowed faces of the cell having an elon- 

 gated elliptical outline), an annular rim, which, growing gradually inwards, 

 constricts the contents of the cell by an annular fiuTow, in a manner exactly 

 similar to that of the commencement of cross -division in a cell of CJadopliora. 

 AMien a cell in this state is treated with substances producing slight endos- 

 mosis (for instance, a weak solution of carbonate of ammonia), the contents 

 retract on both sides from the annular rim, and constitute two completely 

 separate cell-Hke stnictiuTS (halves of a primordial utricle), each of a very 

 long ellipsoidal form, and each lying close agaiust one of the primaiy sides 

 {faces of halves) of the cell. I^Tien the annular rim has grown inwards to 

 about the sixth part of the shortest diameter of the cell, its development is 

 arrested. In natural conditions, this stage is succeeded by the retraction of 

 the primordial utricle from it. Each of these halves of the cell-contents 

 becomes clothed, on the side turned away from the primary side of the cell, 

 with a new membrane, which soon exhibits the fii\st indications of the pecu- 

 liar thickening ribs and nodules of one of the primaiy sides of our Pinnularia. 

 The cell has now completed its division. Seen from one of the secondary 

 sides, it contains two new individuals, equal to the mother-cell in length and 

 breadth, but only possessing one-third of its thickness. The externaUy- 

 situated primaiy side of each of them is the old piimaiy side of the mother- 

 cell, to which we must imagine the newly-formed membrane of the daughter- 

 cell closely adherent at all points. Perhaps the narrow secondary sides of 

 the new cells may be in the same condition. But the contiguous primary 

 sides of the daughter- cells are totally new structures, which, developed rapidly, 

 in a short time become similar to the old primaiy sides in every part. The 

 two daughter- cells are at first held together by the broad middle piece of the 

 secondary sides of the mother- cell, bearing the above-mentioned annular rim 

 inside. The contents of the intermediate space consist of a transparent fluid 

 destitute of any solid structures, doubtless pure water. The two daughter- 

 cells are finally set free by the gradual ' weathering ' of the zone-membrane 

 which holds them together. The division of Surirella hifrons takes place 

 exactly in the same way. An essentially similar kind of vegetative multipli- 

 cation is widely diffused, if not general, in the Diatomese. The well-known 



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