210 Mr J. M. Macffirlane on the 



scarcely imagine two substances of similar clieraical consti- 

 tution, the denser of which had a pellicle formed round it 

 while inside the lighter. The nuclear membrane, in truth, 

 in Spirogyra at least, must be considered to be sometliing 

 as carefully constructed as any other part of the cell ; in 

 other plants, or in cells whose nuclei are always more or 

 less surrounded by protoplasm, it may not be found requisite 

 thus to view it, but in the case before us its importance is 

 too great to be lightly estimated. 



ih) The cellulose septum in being laid down is earliest 

 seen as a clear pale thread of more highly refractive material 

 than the protoplasm, running through the middle of the 

 granules, and compassing the entire circumference of the 

 cell. As I shall afterwards proceed to prove, it results 

 as a secretion from the protoplasm, the edge of it when 

 formed being laid up against the cell-wall all round, and 

 plastered on to it by some agency unknown. Growth is 

 carried on centripetally, so that as additions are being made 

 to it the protoplasm bends in all round, and thereby carries 

 inwards the granules as well as the chlorophyll bands. The 

 septum, therefore, from being a mere thread, soon deepens 

 into a ring, and next into a shelf or ledge running round 

 the interior of the cell, the edge of it now touching the 

 cell-plate (fig. 11a), the consideration of which we will now 

 take up. 



(c) About the time that the first faint indication of the 

 commencing septum is visible, it can occasionally be seen 

 that a splitting is taking place in the middle of the fibres 

 of the nuclear barrel; in others again further advanced there 

 are two lines of granules separated from each other by a clear 

 space. How these granules are formed I am unable to say. 

 It may be that the broken ends of the fibres coil up, or 

 even have material accumulated at their ends to assist in 

 forming the septum; however it be, I am strongly inclined 

 to think, both from their appearance, time of formation, 

 connection with the septum, and optical properties, that 

 they perform in the cell-plate exactly the part played by 

 the granules in the protoplasm, in the formation of the 

 septum ; in other words, that the granules in the cell-plate 

 are present for the same end as are those in the protoplasm. 

 Here we may appropriately notice the result of the 



