1898] THE MOVEMENT OF DIATOMS 413 



to the exterior, so that no structural feature can then be detected even 

 by the best apochroniatic objectives, would seem highly improbable 

 to say the least. The formation of the thread-like prolongations 

 originating at the central node would, on Midler's hypothesis, 

 appear to involve a considerable waste of living substance during 

 prolonged movements ; 1 this could scarcely be brought into harmony 

 with the economy of a single cell, and no proof is given in support 

 of his assertion that the main streams of cytoplasm return to the 

 interior of the cell by way of the canal of the central node. On the 

 whole, according to Lauterborn, everything tends to show that the 

 main streams (gs.) consist throughout of jelly which is driven out 

 at the ends of the cell through the openings of the terminal nodes 

 (especially the ' crescentic polar clefts ' of M tiller), moves in the 

 raphe towards the centre, and projects laterally over it to some 

 extent. The fact that larger fragments of Indian ink or granules of 

 carmine sunk in this jelly are carried in it along the raphe towards 

 the centre proves that the entire hyaline border is actually in motion 

 throughout, but the precise mode of formation of the gelatinous 

 threads and their relation to the rest of the streaming substance is 

 acknowledged to be obscure. 



In contrast to the conditions existing in some species of the 

 genus Pinnularia, there are other diatoms, e.g. Pinrvidaria oblonga 

 aud members of the genera Navicvla, Phurosigma, and Nitzscltin, 

 in which a gelatinous envelope and thread-like prolongations are 

 apparently wanting. In these, the small grains of Indian ink 

 come into contact with the siliceous frustule, and are often moved 

 actively about close to the raphe, as described by numerous ob- 

 servers. So far, the most careful examination has failed to show 

 anything projecting from the raphe, and it would therefore seem 

 that the substance which causes the movements of the foreign 

 bodies, moves within the fissures of the cell-wall, whether occurring 

 in the alae of Surrirella, the keel of Nitzschia, or elsewhere. 

 Lauterborn goes at length into the question whether a substance 

 streaming within a narrow cleft, and so touching the surrounding 

 medium only with a narrow linear portion of its surface, could 

 effect the locomotion of the entire cell, and concludes that a 

 sufficiently powerful force might be developed to overcome the 

 frictional resistance of the surrounding water, pointing out that 

 a somewhat similar principle (the so-called ' hydraulic reaction ') 

 has been successfully employed to propel large ships. 



Owing to the impossibility of examining the substance presumed 

 to be streaming in the raphe, one cannot say whether it is proto- 

 plasm or a gelatinous material, and it must be conceded that the 



1 Midler has since suggested that these prolongations may consist of granules only 

 (smoke-streak appearance), but a connecting substance seems necessary in order to ex- 

 plain certain features observed by Lauterborn. 



