Movements 101 



and Onderdonk (1885) also regarded it as due to an external movement 

 of protoplasm, but Mereschkowsky (1880) concluded that the evidence was 

 in favour of Nageli's theory of osmotic currents through the cell-wall. It 

 was O. Miiller in 1889 who first realised the significance of the pores in the 

 central nodule and the polar clefts at the ends of the valves, and who showed 

 that the raphe in the larger species of Navicula ( Pinnularia) was really 

 a rather complex cleft. He also demonstrated a streaming movement of 

 the protoplasm in the raphe, and ascribed the movements of the cell to 

 the reaction of the motive forces of this living stream of protoplasm upon the 

 surrounding water. Schilberszky (1891), from observations on Synedra, 

 agreed with Pfitzer that the movement was due to an outer coating of 

 protoplasm which escapes from the raphe and is in a state of vibratile 

 motion. He believed that the currents along the raphe were usually inter- 

 rupted jerking or pulsating movements. 



Cox (1890) revived the idea of a line of cilia along the raphe, and 

 suggested that the absence of silica along this line could be accounted for by 

 the obstruction of the moving cilia. Biitschli (1892) also imagined that the 

 presence of a cilium or a fine flagellum would explain the phenomenon, but 

 such structures have never been demonstrated. 



The movements of some of the larger species of Navicula ( Pinnularia) 

 have been stated by Btitschli (1892) and by Lauterborn (1894) to be due to 

 the production of a delicate mucilaginous filament which is protruded from 

 the raphe at some point close to the central nodule. The whole cell is 

 enveloped in a distinct mucilaginous mantle, and both this and the protruded 

 filament are colourless and transparent. The filament is described as 

 elongating by a series of jerks, and Biitschli put this forward as the expla- 

 nation of the jerky movement of diatoms, the cell being pushed backward 

 by the elongation of the filament, the distal end of which is fixed to the 

 substratum. 



In 1893, O. Miiller again emphasized his previous statements relative to 

 the movements of diatoms, and contested the views put forward by Biitschli 

 and Lauterborn. In 1894, Lauterborn affirmed that the production of 

 motility by the streaming of protoplasmic currents would be an isolated 

 phenomenon in either the vegetable or animal kingdom, whereas movements 

 are known to occur in the Desmidiacese and Oscillatoriacese 1 as a result 

 of the excretion of mucilage. Miiller replied in 1894 to the criticism of his 

 hypothesis, and stated that the analogy which had been drawn between the 

 movements of diatoms and of desmids was a false one. The cytoplasmic 

 stream described by Miiller is partly indicated in the accompanying diagram 

 (fig. 74). Supposing the diatom to be moving in one definite direction, 



1 The movements exhibited by various genera of the Oscillatoriacese may not be due to the 

 excretion of mucilage ; see page 21. 



