150 ON RECENT RESEARCHES IN THE DIATOMACE E. 
to such the characters of the cell-contents must be altogether unknown. 
There are others, it is to be feared relatively few in number, who, not 
condite details of structure. Such researches are attended with con- 
siderable difficulty, in consequence of the normal condition of the cell- 
con being ted by circumstances, and the application of reagents 
serving of attention. . 
It is necessary to premise that the views here expressed have special 
reference to the Naviculacee, but are, with some modifications, more or 
Attention is first directed to the Plasm-sac (Plasmaschlauch), consisting 
of a fine colourless plasm forming a closed sac of the shape of the cell, 
produce an immediate contraction of the sac, which at first, as it recedes 
from the cell-wall, preserves the form of the cell, and still maintains con- 
nection with it by a few pellucid threads, but after some time becomes 
contracted into a round mass. This result is brought out most effectively 
per cent., may be satisfactorily preserved in a mixture of glycerine, alcohol, 
d water. This mixture should be applied to the edge of the covering- 
glass, and, after being allowed to evaporate, applied again. He urther 
gave the most accurate description of it in the case of Plewrosigm™ 
is mid asma-mass is discoverable on the side view in both — 
and forms generally but not universally a sort of irregular quadrang e 
Vacuoles and oil-globules oceur imbedded in the plasm, and appear the 
tinetly in consequence of their strong refractive power. Here also, ™ th 
case of the larger forms, Pinnularia, for instance, have been observed "ld 
more or less distinctness short, dark interrupted lines which, 1n gt Z 
pairs, pervade this central mass of plasma, but are most abundant 170 
the centre to the cell-wall. These lines exhibit the same aspec 
positions of the frustule, and therefore are supposed by the author (o s 
cylindrical threads of a thicker consistency than the remainder of the pus jg 
and, perhaps, analogous to the plasm-threads, discovered by Hofmels 
in the plasmodia of Aithalium. in 
A third detail worthy of notice is the central vesicle, which poem 
the middle of the plasm-mass to which attention has just been direct 
