1898] THE MOVEMENT OF DIATOMS 413 
to the exterior, so that no structural feature can then be detected even 
by the best apochromatic objectives, would seem highly improbable 
to say the least. The formation of the thread-like prolongations 
originating at the central node would, on Miiller’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 Miiller), 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, eg. Pinnularia oblonga 
and members of the genera Navicula, Plewrosigma, and Nitzschia, 
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 Niétzschia, 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 Miiller 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. 
