RECENT RESEARCHES IN THE DIATOMACE^. 245 



maiiulcr of the mass, and, perhaps, analogous to the plasm- 

 threads, discovered by Hofmeister in the plasmodia of 

 jEthalium. 



A third detail worthy of notice is the central vesicle, which 

 occurs in the middle of the plasm-mass to which attention 

 has just been directed. It is not equally distinct in all the 

 species of the group, nor in all the individuals of the species. 

 It is very prominent in the Pleurosigmata, in the Naviculce 

 proper, in Siauroneis phoenicenteron, P'mnularia tahellaria ; 

 and though in some cases, even with the most skilful 

 management, it cannot be discovered, our author considers 

 that nevertheless the statement of Luders may be correct, 

 that no Bacillaria-ceW is destitute of such a vesicle, because 

 in many cases, when no such structure can be detected by 

 the ordinary means, even with most careful examination, the 

 application of reagents renders its presence manifest. Dilute 

 hydrochloric acid is the reagent recommended by Schiilze as 

 the most satisfactory for this purpose. 



In addition to these details of internal structure already 

 referred to, a fourth is indicated by Dr. Pfitzer, namely, two 

 plates of endochronie which occur in the cell of the Navicula, 

 of a thick substance and of a yellowish-brown colour. These 

 endochrome plates vary in shape, conformably with the out- 

 line of the siliceous epiderm ; but , in all the species they 

 correspond in these particulars, namely, that they lie upon 

 the connecting bands, and also that they pervade both valves, 

 leaving a small space down the middle free. They do not 

 extend to the extreme ends of the cell, and are more or less 

 constricted at the centre ; their colour varies from light 

 yellow to dark yellowish-brown, and is of the same shade 

 throughout in each case, though varying in different speci- 

 mens. These plates consist of a plasmatic substance differing 

 in density from the plasma forming the sac and the middle 

 mass. In case the normal condition of the cell-contents be 

 disturbed by fracture of the siliceous epiderm, the endo- 

 chrome plates go together, and never commingle with the 

 material of the plasm-sac. If the colouring matter be dis- 

 charged by alcohol, the demarcation of the endochrome-plates 

 from the rest of the plasm can be distinguished. 



As to the colouring material, it has been proved by several 

 authors to be a combination of chlorophyll and a yellowish 

 substance, called diatomin or phycoxanthin, and is similar 

 to the yellow colouring material found by Millardet in the 

 Fucoida. 



In addition to the parts already spoken of, the author has 

 observed in the cells of Naviculce, as well as in all other 



