Mitosis and chromatophores 95 



of chromosomes (consult fig. 68). After division, and on the reconstruction 

 of the daughter-nuclei, the centrosome lies in a slight hollow of the nucleus. 

 Heinzerling states that nuclear division is always mitotic. 



In a somewhat denser mantle of cytoplasm surrounding the nucleus 

 are a number of curious, small, bar-shaped structures disposed in pairs, 

 the function of which is unknown. 





:,_ 



w 





Fig. 68. Stages in the mitosis of two diatoms. Upper three figures represent three stages of 

 nuclear division in Nitzschia sigmoidea (Ehrenb.) W. Sm. Lower figures are of Navicula 

 ( Pinnularia) oblonga Ku'tz. ; the left-hand figure is of a resting nucleus with a large 

 nucleolus. (After Lauterborn.) 



THE CHROMATOPHORES. The chromatophores of diatoms vary much 

 in the different tribes and families. One or many may be present in 

 each cell ; they may be small and discoidal, large and plate-like, or extensive 

 anastomosing structures occupying a large part of the living layer of cyto- 

 plasm. They are frequently very irregular in form, and are sometimes 

 band-like, much lobed, or they present the appearance of perforated plates. 

 In cases where the interior of the cell is divided up by incomplete partitions 

 lobes of the chromatophores generally extend into the chambers. 



