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POPULAR SCIENCE REVIEW. 
raphe, and particularly near the umbilicus and extremities. 
In Fig. 3, which represents a specimen after immersion in 
hyperosmic acid, it is seen that the coloured bands are 
somewhat removed from their natural position. The next 
objects which strike the eye in examining a living Pleu- 
rosigma are highly refractive oil-globules. These are four in 
number ; one pair near either end of the Diatom. They are 
not, however, all in the same plane, one globule of each pair 
being nearer the observer than the other ; their relative posi- 
tion is best seen when a view of the narrow side of the 
frustule can be obtained, so that one raphe is to the left 
and the other to the right, as in fig. 2. The blue-black 
colour, which is assumed by these globules after the Diatom 
has been treated with hyperosmic acid, demonstrates that 
they consist of oleaginous matter. The middle of the cavity 
of the Diatom-frustule is occupied by a colourless finely gra- 
nular mass, whose position in the body is not so clearly seen 
in the flat view of figs. 1 and 3 as in the side view of fig. 2. 
Besides the central mass, the conical cavities at either end of 
the siliceous shell are seen to be filled with a similar granular 
substance, and two linear extensions from each of the three 
masses are developed, closely underlying that part of the 
shell which is beneath the two raphse ; so that in the side view 
(fig. 2) they appear attached to the right and left edges of 
the interior of the frustule. This colourless granular sub- 
stance carries in its centre, near the middle point of the 
Diatom, a rounded nucleus-granule, which it is not very easy 
to see during life ; but may be easily demonstrated by the use 
of acid. The colourless substance is what, in other Diatoms, 
Professor Schultze has shown to be Protoplasm, or vegetable 
sarcode, and there is no doubt that it is an essential part of 
the organization of these minute structures. The protoplasm 
contains numerous small refractive particles, which hyperosmic 
acid colours blue-black, and proves to be fat. Professor 
Schultze found it exceedingly difficult to determine the exact 
boundaries of the protoplasm, on account of the highly refrac- 
tive character of the siliceous shell, and the obstruction pre- 
sented by the bands of endochrome. In the side view, given 
in fig- 2, the colouring matter is not represented; the proto- 
plasm is seen to extend for some distance as a fine layer, 
projecting from the central mass, and passing beneath the 
raphe on both sides ; but after a shorfc distance it seems to 
disappear : it is exceedingly hard to say absolutely whether it 
is continued beneath the raphe, as the granules of the proto- 
plasm, by which only it can be recognized, are very fine. 
After a short distance, the protoplasm reappears, and is 
heaped up in a considerable mass within the conical termina- 
