30 Olive, Mitotic di visiou of tlxe nuclei of tlie C^^anophyceae. 



count a])pruxiiiiately Iwico the oiigiiial immbor of cliruinatin 

 granules. Sueli a plane of lissioii, however, leaves tliis double 

 thread disadvantageously placcd witli reference to the plane of 

 division of the cell whicli follows. Comparing witli tho process 

 as seen in OscUlatoria and in tho higher plants, we shonld ox- 

 pect to find the subsequent plane of iLssion of the cell length- 

 wise; whereas, in reality, we find it crosswise (figs. 73, 76). For 

 a long time, the writer was at a loss to explain this curions 

 discrepancy, but the Unding of such nuclei as drawn in fig. 71 

 fuvnished the clue to its Solution. Jndging froni such appearanc- 

 es, it becomes evident that the Separation of the chromosonies 

 is accomplished siniply by the pulling, or flowing, of the two 

 spireme threads in opposite directions, the one entering the one 

 daughter cell, and the other being drawn into the other cell. 

 In tig. 73, the daughter chromatin niasses are completely sepa- 

 rated, and we no longer see the elongated thread arrangement ; 

 while in fig. 76, a transverse fission plane has cut in two the 

 daughter cells, which have not yet become rounded off at the 

 cut end. 



AVe see, thus, in this species, two differences which separate 

 GJoeocapsa widely from other Cyanophyceae, — first, is the fact 

 that the cell is cut in two by simple constriction and not by 

 a ring-formed wall: and, second, that here we have an excep- 

 tional phenomenon in that the plane of division of the chromo- 

 sonies is abnormal. While, ordinarily, the plane of fission of the 

 chromosonies is parallel to the subsequent plane of division of 

 the cell, in Glococapsa^ on the other hand, fission is at right 

 angles to the resulting plane of division. 



Sumiiiary of mitosis. 



It appears obvious to the writer that we have at present a 

 number of indisputable facts which j^oint to the process of the 

 division of the cyanoi^hyoeous nucleus as mitotic. And, further, 

 that , although the process may be in some respects rather 

 primitive, the essentials of nuclear division are, in the blue green 

 algae, almost precisely similar to the well known karyokinetic 

 processes seen in the higher plants. 



These facts are as follows: 



(1) Spin die. First, we have in the dividing central body 

 an achi"omatic figure, which consists of a central portion, situ- 

 ated between the groups of separating chromosonies, and of a 

 polar portion, corresponding in position to the mantle fibers, 

 which lead from the chromosonies to the cross walls. The 

 mantle fibers apparently have to do with the ]3^^1ing apart of 

 the divided clii'omatin granules. The complete achromatic figure 

 evidently corresponds to the spindle, although it does not usually 

 have the common spindle sliaj^e. Instead, in the short celled 

 filamentous species, it may have the form of a niore or less 

 flattened, broad-poled disc. In the longer celled algae also, the 

 central body assumes soniewhat the form of the cell in which 



