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The Movement of Diatoms 
(ee paper was suggested by a perusal of Robert Lauterborn’s 
“ Untersuchungen tiber Bau, Kernteilung und Bewegung des 
Diatomeen.” It would be impossible to do justice to a work in 
which the letterpress occupies 165 quarto pages, within the limits 
of a single article. The present communication, therefore, will deal 
mainly with the movement of Diatoms, omitting, for the present, 
any reference to the interesting chapters on the protoplasm and its 
inclusions, and on nuclear and cell division—a portion of the work 
well worth separate treatment. For ready comprehension, however, 
the subject must be introduced by an account of the structure of 
the frustule in Pinnularia and Surrirella. 
Structure of the Frustule in Pinnularia major.—The general 
appearance of Pinnularia is sufficiently well known to render any 
special description superfluous. The chief features visible in a sur- 
face view of the frustule, as shown in Plate III., Fig. 7, are (1) the 
presence of a median longitudinal undulating line, the ‘ raphe,’ with 
terminal and central nodes, and (2) a double row of transverse 
markings, the so-called ‘striae’ or ‘costae. The true significance 
of these structures is brought out clearly by Lauterborn. 
The raphe had previously been interpreted by the majority of 
investigators as an open cleft placing the interior of the cell in 
direct communication with the surrounding medium, and Pfitzer 
also sought to justify this view on various grounds. Flégel, however, 
contended that the cleft was closed on its inner side by a thin 
membrane. In order to settle this point Lauterborn carefully 
examined empty frustules of Pinnularia major, and also transverse 
sections (2-3 ~) of examples fixed with chromosmium and stained 
with haematoxylin. One of these sections, from the region between 
the central mass of protoplasm and the extremity of the cell, is 
shown in Fig. 1. The cell-wall exhibits a marked thickening in 
the neighbourhood of the raphe, which runs as a narrow angular 
cleft from exterior to interior without any distinguishable trace of 
an inner limiting membrane. The appearance of this cleft varies 
considerably according to the position of the section in the series 
(see Figs, 2 and 3), being at some points simply oblique, at others 
variously bent, and this irregularity in shape explains why the aspect 
of the raphe alters with a change of focus. In some sections (Fig. 3) 
