A SIEVE-LIKE MEMBRANE IN LEUCOSOLENIA. 255 
shown in fig. 8, which we shall consider again below, was only 
about 116 « in diameter. The sieve membrane is composed 
of two layers of cells in apposition, but separated by a thin 
layer of jelly (figs. 2,8, 12, 18). These cells have a central 
portion containing the nucleus, and are continued out into 
three, four, five, or even six processes, which unite with the 
processes of other cells, thus forming a network with compa- 
ratively wide meshes. The body of the cell forms a node or 
part of one, but not all the nodes of the network are formed 
thus. Many nodes are formed simply by the union of three 
cell processes. Thus larger and smaller nodes can be distin- 
guished. The former contain usually (not always) one, two, 
or even three (fig. 9, c) nuclei, and are often of considerable 
thickness. In a side view (9, 4) or section (12 a, 12 4, 13) of 
such a node the two cells with the jelly between can be easily 
seen. Round the nuclei are a great number of granules, 
sometimes large, more often very small, which turn black in 
osmic, and make the nucleus hard to distinguish in surface 
views. The best preparations are obtained by fixing with osmic, 
which preserves the shape of the network, and then staining 
with picrocarmine, which removes to a great extent the blacken- 
ing. The nuclei can then be readily seen as small spherical 
clear bodies (fig. 9, a, 6, c), usually with a nucleolus, which is 
not, however, always visible. In osmic preparations not 
cleared with picrocarmine, the opacity of the cell makes it 
almost impossible to see the nucleus (fig. 11, a and 4). In pre- 
parations fixed in absolute alcohol the nuclei show up well 
after staining, but the network seems to shrink a little. From 
these larger nodes radiate out the fine strands composing the 
network. Each strand is composed of a very fine core of jelly 
coated by a delicate prolongation from a granular nodal cell. 
The smaller, usually triangular nodes are, as already stated, 
simply formed by the confluence of the fine strand making up 
the network, and have the same structure. The meshes of 
the network are approximately equal in size in different mem- 
branes; in large oscula there are more openings, in smaller 
ones fewer. 
