the Development of the Organic Cell. 9 



towards its middle line. These have, however, as yet such thin 

 and delicate walls, that they are not themselves visible, but become 

 evident only by the changes they produce in the disposition of 

 the layer of chloi'ophyll, and by the action of diosmotic fluids. 

 Moreover, the chlorophyll-deposit gravitates towards the centre 

 of the cell, in the vicinity of the nuclear cell and the approxi- 

 mating ends of the two endogenous cells (figs. 9 & 10). Ac- 

 cording as the enlargement of the nuclear cell, the absorption 

 of the chlorophyll, or the increase in size of the daughter cells 

 prevails at this stage, the subsequent phenomena vary. 



Within the nuclear cell two new cells originate, not by any 

 constriction and segmentation of its membrane, but by new 

 growth within its fluid contents, the nuclear vesicles of which 

 become at the same time absorbed. These facts cannot be more 

 readily verified than in the larger species of Spirogyra. (See 

 Histological Researches, figs. 83-85.) 



The nuclear cell of S. Hornschuchii and of allied species is so 

 delicate and transparent that it cannot usually be clearly demon- 

 strated without the addition of iodine (PI. I. fig. 5). And as 

 the two daughter cells (fig. 6) contained in it are also only ren- 

 dered perceptible by this treatment, this species is therefore not 

 suitable for the study of this histological element of the joint- cell. 



The growth of the two daughter cells contained in the nuclear 

 cell into new joint-cells, as observed in other Spirogyra, does 

 not appear to take place in S. Hornschuchii ; for in this example 

 it is only the daughter cells of the secondary cell which consti- 

 tute new joint-cells, whilst the daughter cells of the tertiary 

 cell (the cell-nucleus) enclosed between septum-building young 

 joint-cells, proceed to grow into the little circular folds on the 

 partition- wall. 



The hypothesis I first promulgated in Wiegraann^s ^ Archiv ' 

 for 1843, on the origin of the cup-shaped circular folds of the 

 septa, derives confirmation from the occurrence of such develop- 

 mental phases as are seen in fig. 11, which, though, indeed, of 

 rare occurrence, are sometimes encounteredj particularly in cul- 

 tivated plants. 



In this example the middle portion of the septum pi'ojects in 

 the form of a hemisphere from each aspect, instead of two cir- 

 cular folds. Between this extreme modification and the usual 

 form every intermediate grade may be met with. 



The precipitation of the chlorophyll on the nuclear cell is 

 scarcely observable under the ordinary conditions of nutrition and 

 development, and is probably connected with a general increase 

 in the amount of chlorophyll deposited. 



It not unfrequently occurs that three daughter cells are simul- 

 taneously developed in a single joint-cell of Spirogyra. In the 



