The Structure of Diatoms. By Dr. J. H. L. FUgel 507 



out so cautiously that the collodion films do not separate ; very 

 careful watching of the imbedding process is therefore necessary, 

 especially also to avoid cracks which commence at the edge and 

 might easily extend to the object. From this bundle, sections may 

 be made according to the method described by me in 1870. 



(2) Any one wishing to study the structure of the individual 

 valves and mark their manner of combination can shorten this 

 somewhat detailed process. A cluster of diatom valves is taken 

 out of the alcohol and placed in a large drop of water on a slide ; 

 and the water is allowed to evaporate after the valves have been 

 evenly distributed. A drop of gum already dry— if possible with 

 a flat surface on a piece of cardboard — should be in readiness ; 

 another piece of glass is coated with oil of turpentine, which is 

 allowed to run off so that a very thin film is left, which does not 

 readily dry. In this the point of a fine needle is dipped vertically, 

 taking up sufficient oil so that by touching a frustule lying on 

 another slide it will adhere. Thus the frustule can be put on 

 the hardened drop of gum which has been moistened by the breath ; 

 this is repeated with a number of valves ad libitum, and finally 

 they are covered with minute drops of gum till the required 

 thickness is attained. This transference of dry frustules upon 

 the dry gum is much easier than the process with fluid gum 

 described under (1), because, with the latter, it frequently occurs 

 that in bringing a new frustule into place the others are disturbed. 

 With uninjured frustules process (2) is not available, because 

 these, after the drying of the thin upper layer of gum moistened 

 with the breath, will at once become charged with air and baflle 

 any cutting. This absorption of air can only be avoided by trans- 

 ferring the frustules from the alcohol directly into the fluid gum, 

 which then diffuses equally through them. At the most the 

 frustule at the moment of hardening is slightly compressed, which 

 injures somewhat the appearance of the sections. 



Mahing Sections. — Numerous attempts to cut diatoms on the 

 microtome failed, and I always returned to cutting by hand, under 

 a dissecting Microscope. Gum is not favourable as the imbedding 

 medium for the microtome. If a better medium were discovered 

 (paraffin is useless), then a new era would open for these researches. 

 Knives with broad backs should be employed ; the angles of 

 inclination to the cutting edge I have used are 21^ 20'. 



Filing-uj) of the Sections. — Pfitzer (19, p. 42) formerly pro- 

 posed to moisten by the breath the gum-chips containing the 

 diatom sections after they had been put upon the slide, whereby 

 they naturally adhered. Anything more unpractical cannot be 

 imagined ; the very thing one wishes to avoid — namely, the dis- 

 turbance of the sections — is by his method certain to occur, and in 

 the most favourable case we have a hardened drop of gum in 



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