276 SUMMARY OF CURRENT RESEARCHES RELATING TO 



takes the form of an axile band uniting the ends of the cell, and con- 

 cealing the nucleus in its swollen central part, which no longer touches 

 the shells ; the nucleus is now held by delicate threads of protoplasm 

 which spring from the plates of endochrome. 



While the axile band is developing thus, the plates of endochrome 

 broaden to such an extent that they almost cover the entire sides of 

 the girdle-bands. At this period, or even earlier, the plates, which 

 were at first constricted, are now bisected. 



At this stage of development the division of the nucleus com- 

 mences. The species under examination exhibits this peculiarity, that 

 the division of the nucleus, which now becomes of a broadly elliptical 

 form, and breaks up immediately into two daughter-cells, proceeds in a 

 direction parallel to the new shells. Only in a few cases are there 

 spindle-fibres stretched between the two daughter-nuclei ; these fibres 

 are knotted in the middle, which may be regarded as a tendency 

 towards the formation of nuclear plates. The daughter-nuclei may 

 divide again, so that in the middle of the axile band there are formed 

 at length from four to seven nuclei. 



When the division of the nuclei is complete, the substance of the 

 axile band becomes firmer, commencing at the ends, and dark dots are 

 seen in it arranged in longitudinal rows, and later, short transverse striae 

 corresponding to them, which, however, become gradually indistinguish- 

 able towards the centre of the band where the nuclei lie. The daughter- 

 nuclei, which hitherto lay one over the other, now approach one 

 another in a horizontal direction (in the transverse diameter of the 

 cell), and are separated only by the axile band, which is continually 

 becoming denser, and which now divides the cell into two halves as an 

 extremely delicate and flexible septum. The new septum becomes 

 further differentiated, splits, and from it are formed the new shells 

 of the daughter-cells. This process goes on very rapidly, and the 

 transitional steps can be readily followed from the simply punctated 

 or striated lamellae to the double lamellas. Cells with split septum 

 are more often met with than with simple septum ; but still more often 

 the septum shows the dots only at the ends. These dots or striae — 

 since the septum is turned with its narrower side towards the observer 

 — probably correspond to the channels of the new shells. 



The new septum now acquires firmness, and now the movement 

 commences of the lamellae of endochrome ; of those which lay upon 

 the old shells, those which were opposed diagonally, with their ends 

 pointing towards the middle of the cell, creep slowly through the sides 

 of the girdle-bands, while the other lamella remains upon the old 

 shell, pushing itself beneath it. Delicate threads of protoplasm are 

 not unfrequently found between the upper ends of the lamellae and the 

 ends of the cell. The lamellae attain their full size very quickly after 

 their transfer. Frequently they break up into two, three, or more 

 pieces, which become transported in the same way as the larger ones. 



The nuclei may break up in the same way by repeated constriction 

 into a large number of daughter-nuclei ; four or five nuclei are not un- 

 frequently found in each daughter-cell. The cells of Synedra Ulni are 

 therefore multinucleated during and after division. 



