432 Prof. H. Karsten on the Formation, 
monia on the septum, when in course of thickening, and while 
apparently still simple, the limits of the two daughter cells 
forming it are distinctly seen; in other words, its construction 
out of two lamelle is demonstrated. Iodine, when applied im a 
concentrated state, has a more energetic solvent action upon the 
recently thickened primary cell-membrane ; and when its solu- 
tion is diluted, it is at the same time exosmotic in its action on 
the contents of the secondary cells. 
If these solvents be allowed to act for a time upon half- 
lignified septa, the different component cell-membranes of which 
have been separated from each other as much as possible by 
means of diosmotic fluids, the newly lignified septum thus set 
free is eroded at the centre, and finally more or less dissolved, 
whilst the two contracted and mutually adherent daughter 
cells float freely in the cavity of the parent cell. Examples 
such as that represented in fig. 67, of a Spirogyra treated 
with glycerine and afterwards with a solution of iodine, should 
therefore be cautiously used in elucidation of the mode of origin © 
of a septum. 
In this experiment it now and then happens that one young 
joint-cell (as shown in PI. VII. fig. 67) which is adherent by one 
extremity to its neighbouring sister cell, and with it detached 
on every side from the mother cell, is at the same time still 
attached to the opposite end of its mother cell, by which a con- 
siderable tension of the young cell in its long diameter is pro- 
duced, and being transferred to the neighbouring septum, is 
exerted chiefly on the delicate young wall in process of lignifica- 
tion. This is consequently somewhat drawn into the cavity of 
the extended cell until it becomes lacerated at its point of union 
with it, whereupon it again assumes its flat form, and the second 
daughter cell is drawn through the circular central opening so 
formed in the septum into the adjoining cell-cavity. 
At times the lignified portion of the septum is so delicate 
that, after the operation of a solution of glycerine, only a very 
slender annular thread is visible on the inner surface of the 
mother cell; and it might be supposed that the two adherent 
contracted secondary daughter cells floated freely in the centre 
of the mother cell. Upon the addition of a diluted watery solu- 
tion of iodine, which is greedily taken up by the fluid contents 
of the primary cell, it is perceived, by the movements thereby 
induced within the two contracted and coherent daughter cells, 
that these do not float freely in the centre, but are fixed in their 
position by the thread-like annular septum, which, from the 
cloudiness of the cell-fluid, caused by the solution of iodine, 
appears like an extremely thin clear lamina, and exhibits slight 
movements corresponding with those of the daughter cells. 
