OF THE DIATOME^. 49 



clear vesicle, from which thin, finely granular lines or threads extend and 

 intersect and branch in a reticulate manner, with a more or less distinct 

 radiation, the more fluid contents flowing between them. In the long cylin- 

 drical fnistules of RMzosolenia, on the contrary, these granular mucilaginous 

 threads run longitudinally. Within these threads the colom^ing, yellowish- 

 brown corpuscles, not circular, but, as Schultze says, irregularly multangular, 

 are disposed and retained in their position. Although these researches extend 

 to so few forms, yet we are disposed to believe that this disposition of the 

 elements of the endochrome will be found to be the rule. A regular arrange- 

 ment is figured in many di'a wings of the Diatomeae by various observers ; 

 and where it does not appear it is most probably due to the want of attention 

 to its presence, — or to the excessive multiplication of the coloming corpuscles, 

 causing them to appear spread beneath the envelopes as a pretty uniform 

 layer. A definite disposition of the chlorophyll-granules is common in plants, 

 particularly among the lower Algae, and owes its constancy to the presence 

 of the mucous and less fluid contents, which are condensed from the sur- 

 roimding fluid in the form of filmy threads, and serv^e as a nidus to the colour- 

 ing particles. In this disposition, therefore, of the endochrome and its cor- 

 puscles we perceive a vegetable character, as contrasted with what is seen in 

 animal ceUs, and find in it an additional argument for the vegetable nature 

 of the Diatomeae. 



Notwithstanding that the endochrome is, by pretty general consent, ho- 

 mologous with that of recognized vegetable Algae, still it would seem to be 

 of a different chemical composition as well as of another coloui\ Kiitzing, 

 indeed, insisted on the fact of the similarity of the endochrome to the 

 gonimic substance of Algae, from the circumstance that, by means of alcohol, 

 he was able to extract a coloimng matter similar to chlorophyll ; yet Eaben- 

 horst and others have remarked a difference in chemical nature. Prof. Smith 

 again, whilst admitting the imperfection of our knowledge on this point, 

 goes on to say that " the tincture of iodine causes the internal membrane to 

 contract upon the cell-contents, and converts these from the golden yellow 

 which they exhibit in some species, into bright green, and that a weak 

 solution of sulphuric acid, while it eff'ects the same contraction in the ceU- 

 waU, gives to the contents, which have been previously treated with iodine, 

 a dark-brown hue : alcohol, on the other hand, as in the case of vegetable 

 ceUs in general, dissolves the utricle and its contained endochrome, or at all 

 events entirely removes their colour, and leaves their silicious epiderm in a 

 state of perfect transparency. It does not, however, dissolve the envelope in 

 which the frustules of the frondose forms are imbedded, nor the filamentous 

 stipes or gelatinous ciLshions to which other species are attached." 



Meneghini {op. cit. R. S. p. 365) contends that the identity in nature of 

 the endochrome of Diatomeae and of Algae is not proved. " Its colour is dif- 

 ferent ; and it is diiferently coloured by chemical reagents. The resemblance 

 to it in some instances, as in Melosira, in regard to conformation and suc- 

 cessive alterations, is only in appearance. In the endochrome of Algae the 

 monogonimic substance begins by presenting a granular appearance ; then it 

 becomes distinctly granulated and changes into the polygonimic substance, 

 so minutely described by Kiitzing. But these changes do not occur in 

 the coloured substance of Diatomeae. If we insist on a parallel, we can 

 only compare it to the cryptogonimic substance of Byssoidia, CaUithcimnia, 

 Griffiihsia, and Polysi])ho7iia. It divides into two parts which successively 

 undergo ulterior division ; and in regard to these changes we may observe 

 that there is an essential distinction between those that occur during life and 

 those that take place after death, the greater number happening in the latter 



