ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 891 



ceptible in water, unless it be charged with suspended matter, and 

 then only show their presence by displacing this matter from the 

 space which they occupy themselves." 



Sections of Pinnularia.* — W. Prinz returns to this subject.f 



" In the preceding ' Bulletin ' I made some remarks on a note of 

 Mr. Burgess | as to the nature of the sculpturings of Coscinodiscus 

 Oculus-Iridis and Trinacria regina, with unpublished drawings of the 

 late Walker Arnott, representing the structural details of Pinnularia. 



According to these drawings the striae, more or less at right angles 

 to the raphe which decorates the valves of this diatom, are tubes. § 

 These details were studied on the fracture edges placed in positions 

 favourable for examination. By mere inspection, we can convince 

 ourselves that these natural sections present the same drawbacks as 

 the sections of Pleurosigma obtained by Dr. Flogel, to which they are 

 compared. They are too thick and often give deceptive images 

 particularly with penetrating objectives. 



M. Piitzer || has given a very detailed description of the structure 

 of Pinnularia. It is based on the examination of fracture edges, 

 and especially of sections made with a razor, by a process similar to 

 that of Dr. Flogel, that is to say, with diatoms cemented together 

 with gum. This method does not appear to have given quite satis- 

 factory results, for the drawings which accompany the paper are 

 partly diagrammatic. 



I have attempted by a different process to obtain sections of this 

 diatom, in order to give a more faithful representation of it. I chose, 

 in a good specimen of Franzensbad earth, some small pieces rather 

 more coherent than the rest of the mass. They were boiled in 

 Canada balsam to harden them. After this treatment, they can be 

 thinned by means of emery, in the same way as minerals and 

 rocks. 



This method has, however, the disadvantage of necessitating the 

 use of heat for fixing on the slide the fragment, already ground on 

 one of its faces. The operation must be performed w^ith care and 

 rapidity, to avoid the softening of the balsam which unites the 

 diatoms, in order not to displace or break them. In this way I have 

 obtained thin laminae, of about a square centimetre in surface, 

 containing hundreds of sections at right angles to the long axis 

 of the frustule. Contrary to my expectation, not a single one gave 

 me a clear image of the raphe. These preparations were otherwise 

 irreproachable and of extreme thinness, in spite of the friability of 

 the material. At the place where the raphe and the connectives 

 are the sections had a confused, mutilated appearance, which made 

 their observation very difficult. I thought I saw the thickness of 



* Bull. Soc. Belg. Micr., ix. (1883) pp. 136-42 (4 figs.), 

 t See this Journal, ante, p. 695. 

 t Micr. News, iii. (1883) p. 71,. 



§ This view was also maintained by Schumann (" Diatomeen der hohen 

 Tatra," Verh. Zool. Bot. Gesell. Wien, 1867, p. 73). 



II Pfitzer, * Untersuchungen iiber Bau u. Bntwickelung der Bacillariaceen,' 

 1871. 



3 O 2 



