164
SAUL WISCHNITZER
randomly distributed ribosomes. Particles corresponding to those identified as glycogen have not been seen in amphibian oocytes. This suggests
that the glycogen is present as macromolecules that are dispersed throughout the ground cytoplasm.
D. Discussion
The presence of the yolk nucleus is quite variable in amphibian eggs.
In specimens of Rana pipiens (Ward, 1962) and Triturus viridescens (Hope
et al., 1963; Wischnitzer, 1964a) prepared for electron microscopy, this
organelle has not been seen. It has, however, been observed in Rana
esculenta (Wartenberg, 1962), Xenopus laevis (Wartenberg, 1962; Balinsky
and Devis, 1963), and Necturus maculosus (Kessel, 1963). Thus, the presence
of a yolk nucleus varies not only among the different genera but from species
to species. This observation is consistent with the generalization made by
Sotelo and Trujillo-Cenoz (1957) that in some species organelles such as
mitochondria may be organized into clusters (i.e., yolk nuclei), whereas in
other species they may be dispersed.
The electron-microscope observations of Balinsky and Devis (1963)
concerning the life history of the yolk nucleus are similar to those of
Wittek (1952) on living young oocytes of Rana temporaria. The latter
investigator found this body is closely associated with the nucleus and that
it later moves toward the periphery and disappears.
The function of the yolk nucleus is uncertain. In amphibians, at least,
it is quite probably not related to yolk formation. This is suggested from the
electron-microscope data of Balinsky and Devis (1963) which demonstrate
that platelet synthesis has already taken place in the peripheral cytoplasm
by the time the yolk nucleus reaches this region. Thus the name for this
structure is misleading and Balinsky and Devis, like Ward (1962), justifiably feel that it should be dropped.
Wittek (1952) has suggested that the yolk nucleus is involved in the
production of lipids. Although lipochondria were seen by Balinsky and
Devis (1963) to be at times associated with the yolk nucleus, the relationship was considered to be fortuitous. The conclusion that no significance
is attached to the structural relationship between the lipochondria and
yolk nucleus is based on the frequency and the nature of the morphological
association that was observed between these two cytoplasmic elements.
Raven (1961), in his review, has suggested that the migration of mitochondria, as the yolk nucleus, to the oocyte periphery might serve as a
specialized mechanism of transfer for nuclear information. This view is,
in a broad sense, in harmony with the findings of Balinsky and Devis
(1963), who suggested that granular nuclear material is involved in the
organization of the yolk nucleus. Having arrived at their permanent
SAUL WISCHNITZER
randomly distributed ribosomes. Particles corresponding to those identified as glycogen have not been seen in amphibian oocytes. This suggests
that the glycogen is present as macromolecules that are dispersed throughout the ground cytoplasm.
D. Discussion
The presence of the yolk nucleus is quite variable in amphibian eggs.
In specimens of Rana pipiens (Ward, 1962) and Triturus viridescens (Hope
et al., 1963; Wischnitzer, 1964a) prepared for electron microscopy, this
organelle has not been seen. It has, however, been observed in Rana
esculenta (Wartenberg, 1962), Xenopus laevis (Wartenberg, 1962; Balinsky
and Devis, 1963), and Necturus maculosus (Kessel, 1963). Thus, the presence
of a yolk nucleus varies not only among the different genera but from species
to species. This observation is consistent with the generalization made by
Sotelo and Trujillo-Cenoz (1957) that in some species organelles such as
mitochondria may be organized into clusters (i.e., yolk nuclei), whereas in
other species they may be dispersed.
The electron-microscope observations of Balinsky and Devis (1963)
concerning the life history of the yolk nucleus are similar to those of
Wittek (1952) on living young oocytes of Rana temporaria. The latter
investigator found this body is closely associated with the nucleus and that
it later moves toward the periphery and disappears.
The function of the yolk nucleus is uncertain. In amphibians, at least,
it is quite probably not related to yolk formation. This is suggested from the
electron-microscope data of Balinsky and Devis (1963) which demonstrate
that platelet synthesis has already taken place in the peripheral cytoplasm
by the time the yolk nucleus reaches this region. Thus the name for this
structure is misleading and Balinsky and Devis, like Ward (1962), justifiably feel that it should be dropped.
Wittek (1952) has suggested that the yolk nucleus is involved in the
production of lipids. Although lipochondria were seen by Balinsky and
Devis (1963) to be at times associated with the yolk nucleus, the relationship was considered to be fortuitous. The conclusion that no significance
is attached to the structural relationship between the lipochondria and
yolk nucleus is based on the frequency and the nature of the morphological
association that was observed between these two cytoplasmic elements.
Raven (1961), in his review, has suggested that the migration of mitochondria, as the yolk nucleus, to the oocyte periphery might serve as a
specialized mechanism of transfer for nuclear information. This view is,
in a broad sense, in harmony with the findings of Balinsky and Devis
(1963), who suggested that granular nuclear material is involved in the
organization of the yolk nucleus. Having arrived at their permanent