SCHULTZE, ON DIATOMACES. 15 
delicate layer of the same finely granular matter. In the 
latter, as in the fine granular substance enveloping the 
nucleus, and frequently in an incompletely fixed state of the 
wall, entirely concealing it, are imbedded vesicles of a 
coloured material. These are ochre-yellow, and round or 
somewhat pointed; they are of the latter form in Denticella. 
They lie close to the siliceous covering, either presenting an 
entirely uniform distribution at equal distances from one 
another (as m the examples delineated), or are arranged in 
reticulated cords, uniting among each other, as has been 
more than once seen in Coscinodiscus. In the threads and in 
the finely granular external layer are found appearances of 
currents. Specimens brought freshly from the sea, or kept 
at most some hours in a glass, are alone fitted for the ob- 
servation of these. From the finely granular mass sur- 
rounding the nucleus the stream goes in, and to the, as it 
appears, more homogeneous, if not structureless, mass, con- 
sisting of threads at the periphery; and in the same threads, 
or others, different granules return back to the centre. The 
threads are thickest near the nucleus, and attenuate them- 
selves on their way by division, anastomosing reticulately 
with one another, till, in their finer distribution, they repre- 
sent a delicate web lying close to the siliceous coat, in which, 
or in a more homogeneous layer, immediately on its exterior, 
the coloured vesicles are imbedded. The nucleus does not 
always lie close to the siliceous covering; it may also 
approach the middle of the imternal space. Coscinodiscus 
possesses a form like a shallow round box, whose bottom and 
top is arched like a watch-glass. If one views such a body 
from the side, and the nucleus with the granular mass 
enveloping it is situated in the middle, between both side 
walls, there goes more frequently a stronger cord of granular 
matter from it to the centre of the latter. In this way the 
middle appears possessed of a darker axial cord. In such a 
middle situation, the nucleus seems to increase before the 
commencing propagation by division. After the appearance 
of two new watch-glass-shaped walls, with their convex 
surfaces turned to one another, that sprout of division 
possesses a nucleus closely adhering to the newly formed wall. 
So also in Denficella (as fig. 12 shows), where certainly the 
situation of the nucleus is disclosed only by the accumulation 
of the darker granular mass, as the centre of the radiating 
threads. The coloured vesicles are omitted in this figure ; 
they presented the same uniform destribution as in fig. 11, 
so that a complete removal of the same seems not to interfere 
with the appearance of the division. 
