ON RECENT RESEARCHES IN THE DIATOMACEZ. 151 
It is not equally distinct in all the species of the group, nor in all the in- 
dividuals of the species. It is very prominent in the Plewrosigmata, in 
the Navicule proper, in Stauroneis phenicenteron, Pinnularia tabellaria ; 
d in i i 
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 Schülze as the most satisfactory for this 
Tpose. : 
In addition to these details of internal structure already referred to, 
a fourth is indicated by Dr. Pfitzer, namely two plates of endochrome 
which occur in the cell of the Navicule, of a thick substance and of a 
yellowish-brown colour. These endochrome plates vary in shape, con- 
formably with the outline of the silicious epiderm ; but in all the species 
they correspond in these particulars, namely that they lie upon the con- 
necting bands, and also that they pervade both valves, leaving a small 
s to the colouring material, it has been proved by several authors to- 
be a combination of chlorophyll and a yellowish substance, called diatomin 
Millardet in the Fucoidee. : 
In addition to the parts already spoken of, the author has observed. in 
the cells of Navicule, as well as in all other Bacillariacee, two others, 
The oil-globules afford a means of answering the question whether the 
In favour of 
