104 



Bacillariede 



quite conceivable that 'they may not all be due to the same cause. The 

 slow rolling and turning movements exhibited by some forms are very 

 different from the rapid forward and backward propulsion so conspicuously 

 shown by others. Those diatoms with the highest development of the raphe 

 show the quickest movements, generally a jerky propulsion which enables 

 them to move with more or less rapidity from place to place. The move- 

 ments of some of the smaller species of Navicula, and also of Nitzschia, are 

 so rapid and continuous that it is scarcely possible to accept Lauterborn's 

 explanation that they are due to the excretion of mucilage. If that were the 

 case, some of these very active diatoms would have to secrete an incredible 

 amount of mucus in a very short space of time. In such diatoms there is 

 every probability that the movements are caused, as stated by O. Mliller, 

 by the protoplasmic currents circulating in the clefts of the raphe. 

 Moreover, in the genus Nitzschia no mucilaginous threads have been 

 demonstrated. 



The Navicula ( Pinnularia) type of raphe is the most perfectly de- 

 veloped, but other somewhat less efficient types occur among the navi- 

 culoid diatoms belonging to the Gomphonemacese and Cocconemacese. 



CELL-DIVISION. Diatoms are incapable of growth in size owing to the 

 siliceous nature of their cell-walls, but slight alterations in volume can take 

 place by a sliding movement of the connecting band of the older half of the 

 cell over that of the younger half. The usual method of increase is a multi- 

 plication by successive bipartitions, each -cell-division resulting in a gradual 

 reduction in the size of the individuals. A slight increase in the volume 

 of the cell is the first appreciable change, after which there is a mitotic 

 division of the nucleus. The stages in this division have been carefully 

 worked out by Lauterborn ('96) in several large species of Navicula, Surirella 

 and Nitzschia. A division of the protoplast follows immediately on the 

 division of the nucleus, beginning as an infolding in the plane of the girdle 

 of the peripheral layer of cytoplasm. When this infolding is complete, new 

 siliceous valves, at first very delicate, are formed over each divided surface. 

 These new valves are at first situated within the girdle of the original cell, 

 but with the growth of the new valves and the development of their 

 connecting bands, the two connecting bands of the original girdle become 

 separated, each forming one half of the girdle of the daughter-cells. Thus, 

 each individual produced by division consists of a new half and an old half, 

 and the connecting band of the old valve overlaps that of the new valve. 

 Owing to the formation of a pair of new valves within the girdle of the old 

 ones, and since the cells when once formed are incapable of growth, the 

 newer half of every successive generation becomes reduced in size by the 

 double thickness of a connecting band. This statement, however, is not 



