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PACIFIC SCIENCE, Vol. XXII, July 1968 
roundish mass of finely granular material some- 
what more dense than the rest of the nucleus 
but considerably less dense than the first com- 
ponent (Fig. 2). 
In the resting stages, that is, in the stalk cells 
and the spores, the first granular component 
often contains lamellar elements which are 
stacked in parallel fashion (Fig. 4). The num- 
ber of lamellae per stack has been observed to 
vary from 1 to 12. The lamellae can be flat or 
curved, may cross through the middle of the 
granular component (Figs. 4 and 5), or may 
follow its contour (Figs. 6 and 7). Yet they 
never extend beyond the granular mass. The 
lamellae are separated by a constant distance 
of approximately 150 A. The constant thickness 
of the lamellae together with the constant 
spacing conveys to the whole structure a regu- 
larity of appearance (Fig. 4). 
Each lamella consists of a single layer of 
granules (Figs. 1, 6, and 7) apparently iden- 
tical to the granules making up the , first com- 
ponent (Figs. 1, 2, and 8). In fact, the layers 
are often continuous with the granular com- 
ponent (Figs. 8 and 9) and are always closely 
adjacent to it. Oblique sections through the 
stacks provide additional evidence that the 
layers are built from granules and not, for 
example, from a filamentous component (Fig. 
8). Furthermore, we see that the granules are 
not oriented in any particular way but are 
packed rather tightly, though randomly, within 
the planes. 
By applying the histochemical tests described, 
ribonucleoprotein-containing areas can be 
demonstrated within the nucleus comparable in 
size and location with the components described 
at the electron microscope level. This suggests 
that the granular masses contain ribonucleo- 
protein and are probably aggregates of ribo- 
somes or ribosomal precursors. The nucleolus 
has been shown to be a source of ribosomes 
(Birnstiel, Chipchase, and Hyde, 1963). Our 
best evidence supporting this idea is obtained 
from the electron microscope study of RNAase 
treated cells in which the granular component 
has largely faded (Fig. 10). 
DISCUSSION 
The lamellae described in this report bear a 
certain resemblance to the "core” structures of 
spermatocytes (Moses, 1956) as well as to the 
chromatoid bodies in parasitic protozoa and in 
germ cells of plants and animals (Barker, 
1963). In the case of the "cores” a close simi- 
larity can be noted between the outer layer of 
the complex cores described by Schin (1965) 
and our lamellae. In both cases the layers are 
granular, show continuity with the nucleolus, 
and are often attached to or partly surrounded 
by the latter (Schin, 1965; Guenin, 1965). 
In the case of Acrasis, however, all the other 
components of the "core” structure are lacking; 
also, the cells are at a resting stage and no 
division of cells or nuclei occurs. 
In the chromatoid bodies of Entamoeba in- 
vadens, granules identified as ribonucleopro- 
tein particles also are arranged in layers. A 
closer examination of those lamellae reveals, 
however, that the granules within the layers 
form helices (Siddiqui and Rudzinska, 1963) 
and thus differ from the arrangement described 
here for Acrasis. 
All of these examples demonstrate the capac- 
ity of ribonucleoprotein granules to assemble 
into orderly structures. In our case particularly, 
we attribute this to a process of spontaneous 
self assembly within or at the periphery of a 
highly concentrated mass of granules in a state 
of low metabolic activity. 
The fact that the lamellae have so far been 
observed only in resting cells, and then not with 
regularity, indicates that they do not represent 
an organelle involved in an essential function. 
SUMMARY 
Lamellate structures have been demonstrated 
within the nucleoli of spores and stalk cells of 
the slime mold Acrasis rosea. The lamellae, 
numbering up to about 12, are separated from 
each other by about 150 A and are arranged 
in a parallel fashion to form stacks which either 
cross the main nucleolar mass or line it periph- 
erally. Each lamella is composed of a single 
layer of granules apparently identical to the ones 
making up the bulk of the nucleolus. The 
granules are fairly tightly but randomly packed 
within the layer. The lamellate structures are 
interpreted to represent an arrangement of ribo- 
somal type particles formed from a concentrated 
pool of particles by a process of self assembly. 
