December 1898] THE MOVEMENT OF DIATOMS 407 



the walls of the cleft approach one another very closely in such a 

 manner as to simulate the appearance of an internal closure. In 

 those examples in which the cleft, seen in transverse section, has an 

 angular course, 0. Miiller considers it probable that the central 

 portion is closed during life by the middle lamina, in which case 

 two canals would persist, one running along the outer surface, the 

 other along the inner surface of the cell- wall, but Lauterborn 

 has not observed such a closure, nor does he consider its occurrence 

 probable. This is of some importance, since Miiller postulated the 

 presence of such canals in his theory of the movements of diatoms. 



As to the transverse markings (Riefen) of the frustule, Lauter- 

 born's results are entirely in agreement with those of Flogel, who had 

 previously described the appearances as being, in reality, chambers 

 hollowed out in the substance of the frustule, and communicating 

 by a tolerably wide opening with the interior of the cell. These 

 features are well shown in Fig. 1, where the plane of section passes 

 through four of these chambers, and Fig. 2, which is an outline 

 drawing of a section passing between two chambers in one valve of 

 the frustule. These drawings also illustrate the way in which the 

 two valves are united by the overlapping of their free edges. 



The interior of the cell is lined during life by a layer of proto- 

 plasm, thinner laterally than elsewhere, and, in the middle of the 

 cell, forming a transverse bridge-like mass containing the nucleus. 

 On each side of this central portion the peripheral protoplasm 

 surrounds a large vacuole filled with cell-sap. This condition of 

 things suggested to Lauterborn a possible explanation of its meaning, 

 which, although purely hypothetical, is worth consideration. 



The researches of 0. Miiller have shown that the protoplasm of 

 Pinnularia is subjected to a very considerable osmotic pressure 

 (4 to 5 atmospheres), manifested with equal intensity in all direc- 

 tions. Now pressure in the direction of the lateral walls would 

 result in pressing the overlapping elements closer together, but 

 when exerted at right angles to this, in the direction of the raphe, 

 there would be a tendency to force the two halves of the frustule 

 apart, if the frictional resistance of the overlapping elements was 

 not sufficient to maintain equilibrium. But, as we have already 

 seen, on each side of the raphe internally a great number of 

 transverse chambers are placed one behind the other, and filled by 

 prolongations of the peripheral protoplasm that enter each chamber 

 by an opening about half the diameter of its internal cavity. 

 Lauterborn Buggests that these chambers filled with abstricted 



DO 



portions of the protoplasm might be imagined to act as so many 

 clamps (' Klammern ') opposing a strong resistance to the force tending 

 to separate the two halves of the frustule, and so strengthening the 

 frictional resistance of the overlapping elements. He points out 



