426 Mr. C. Mereschkowsky on Stauronella. 
centre, where it forms a more or less well-marked circular 
space, in which the nucleus is placed. The terminal ends 
of the plates may be straight (fig. 17) or provided with a 
more or less deep longitudinal sinus (fig. 16). 
Each plate contains in its centre a conspicuous pyrenoid 
(figs. 16, 17, 19, pr.), composed of a refractive material, 
which is imbedded in the very substance of the chromatophore- 
plate, thus being limited on all sides by the coloured endo- 
chromemass. It formsa hemispherical protuberance projecting 
into the inner space of the frustule, almost reaching its central 
axis (figs. 16, 17), sometimes being separated from the walls 
of the frustule by a hemispherical hyaline space (fig. 16). 
This pyrenoid is, however, only seen when the diatom presents 
to the observer its valve-face; as soon as it is moved to one 
side or the other, even in a slight degree, it immediately dis- 
appears, a peculiarity which also belongs to the pyrenoids of 
most species of the genus Nétzschia. It is obvious that no 
trace of pyrenoids will be seen when the diatom presents its 
girdle-face, as in fig. 18. In this figure the centre of each 
plate, 2. e. the centre of the lighter part of it, is the real place 
of the pyrenoid, and there it ought to be seen had it not 
become invisible. Such an absence of a visible pyrenoid 
in the girdle-face is also a characteristic of most of the 
Nitzschie. 
The eleoplasts are round, of a comparatively large size, 
variable in number, placed at the outer and inner ends of the 
chromatophore-plates (fig. 18). The colour of the endo- 
chrome is yellow-brown. 
As above described, the endochrome of Stauronella con- 
stricta appears to be of a very peculiar nature. In order, 
however, to understand in what this peculiarity consists, and 
so to appreciate its whole value, I shall have to enter into 
some general considerations concerning the endochrome of 
diatoms based on my rather extensive studies on this subject. 
I have carefully studied the endochrome of over three 
hundred forms, amongst which not less than one hundred 
and fifty belong to the Naviculacezee and Nitzschiee. As 
a general result of the comparison of the endochrome of 
these two groups, I can state that the endochrome of the 
Naviculacee is of a quite different and even diametrically 
opposed type to that of the Nitzschiee. In both groups the 
typical number of chromatophores is two; but while in the 
Naviculacee these plates have a longitudinal disposition, 2. e. 
they are placed side by side and separated by a longitudinal 
interval along the longer axis of the frustule, each plate 
