408 NATURAL SCIENCE [December 



that this hypothesis would gain in probability if it could be 

 proved that the protoplasm was withdrawn from the chambers 

 when the two halves of the frustule parted from one another 

 during cell-division, a circumstance which he regards as by no 

 means improbable. Pfitzer does not consider the hypothesis 

 summarised above very probable, but believes that the chambers 

 with their very thin outer walls would probably relieve the osmotic 

 pressure ; Lauterborn, however, seems justified in his contention 

 that the two functions in no way exclude one another. 



Structure of the Frustule in Surrirella calcarata. — This 

 diatom is one of the largest and most beautiful fresh-water forms. 

 Examples measuring 03 mm. in length and - 2 mm. in lateral 

 breadth were not uncommon, so that, under favourable conditions, 

 they could be readily perceived with the naked eye. 



Seen in transverse section (Fig. 4) the outline is roughly 

 rectangular, with two sides (' Gurtelseiten ') flattened, and the 

 other two (' Schalenseiten ') more or less hollowed out on each 

 side of a median ridge or keel. At each corner is an outstanding 

 process {ala, ' Fltigel '). These processes vary in appearance with 

 the plane of section (see description of Plate) and three aspects are 

 shown in the figure, combining features exhibited by different 

 sections. The drawing also illustrates the complicated character 

 of the chromatophores, and their relation to the central bridge-like 

 mass of protoplasm containing the nucleus. When viewed in its 

 entirety, and with one of the lateral surfaces (flattened sides in 

 section) turned towards the observer, the diatom presents the 

 appearance of a more or less broad wedge, the median portion 

 occupied by the lobed chromatophores (superficial focus) and 

 flanked on each side by the alae which are now seen to consist 

 of a number of parallel transverse canals connected at their 

 extremities by a longitudinal canal, and separated from each other 

 by tolerably wide interspaces. In a surface view (' Schalenseite ') 

 the contour of the frustule is almost lanceolate with a median 

 longitudinal ridge produced at either end into a spur-like process, 

 the anterior one being the largest. 



To return to the alae, Figure 5 shows a portion of one of these 

 wing-like processes viewed from the surface. Along the edge, for 

 its whole extent, passes a rather narrow canal (' Fliigelrandkanal ' or 

 ' Fliigellangskanal ') a connection being established between this and 

 the interior of the cell by numerous short transverse canals. 

 Internally, these latter commence as tolerably wide tubes of rounded 

 section ; towards the longitudinal canal (lc), however, they steadily 

 diminish becoming more elliptical in section. The transverse canals 

 are separated from one another by U-shaped intermediate pieces, 

 formed of apposed portions of the cell-wall, whilst the spots where 



