610 Transactions of the Society. 



treated by different methods, viz. (1) by the section method, (2) 

 the east method, (3) the staining method. 



§1.1 commence with the section method, remarking that most 

 of my sections were made as described under (2). It is requisite 

 that investigators should keep strictly to this method, otherwise 

 they will not see the details of structure here described, or they will 

 obtain from oblique sections images very difficult to interpret. We 

 require very thin (e. g. 1/1000 mm.),* exactly transverse and longi- 

 tudinal sections, whilst with Pleurosigma it does not matter if the 

 direction of the cut deviates more or less from a right angle to the 

 midrib. The transverse section of a valve of Pinnularia, if it has 

 not touched the central nodule, has either the form of fig. 1 or fig. 2, 

 plate VIII., except that in close proximity to the two ends the general 

 form, by the disappearance of the rounded-off right angle, becomes 

 semi-circular or semi-elliptical. In order to elucidate the change in 

 the appearance of the inner structure we must remember that 

 the surface image of Pinnularia exhibits coarse transverse striae, 

 ending near the midrib, and which are regarded by Pfitzer and 

 others as superficial furrows, and they have therefore been designated 

 by various improper terms, such as furrows, surface sculpture, &c. In 

 reality the outer surface^ independently of the midrib, has neither 

 elevations nor depressions, but is quite plane. If the section passes 

 through the middle of a so-called furrow, it appears as in fig. 1 ; 

 if it passes through the interspace, then it appears as in fig. 2. The 

 separate parts of the image are, as will be shown later on, to be 

 explained as follows : — Each so-called furrow is an inner chamber 

 of the membrane, and, in proportion to the chambers of Pleurosigma, 

 of enormous size, since it extends from the edge of the frustule to 

 very near the midrib. It is also of almost equal thickness through- 

 out. But what is most remarkable is the fact that each chamber 

 has a rather broad opening on the inner side of the cell-wall, by 

 which it can be readily examined. The outlines of the opening are 

 easily observed in the surface view, and have been often represented 

 in the better class of illustrations {vide Pfitzer, 19, pi. 1, fig. 2). 

 The draughtsmen, however, do not seem to have had a clear con- 

 ception of the signification of these lines. Pinnularia is represented 

 as a definite proof that the cell-envelope is broken through in the 

 midrib, thereby allowing free exit to the protoplasm. All former 

 reliable researches having apparently proved the non-existence of 

 openings, and the endosmotic process having been generally accepted 

 as the cause of movement {vide Naegeli, Von Siebold, W. Smith, 

 Eabenhorst), Prof. Max Schultze (24) in 1865 put forward the 

 opposite view and considered it proved that at the raphe of the 



* Sections as made by Pfitzer (19, p. 43) which are twice or thrice the thick- 

 nees of a furrow, cannot, as a matter of course, be used for a delicate observation. 

 At the best one recognizes only the general outline. 



