196
SAUL W I S C H N I T Z E R
berg, 1962). Such changes apparently reflect the active participation of
the nucleolus in the physiological processes taking place in the cytoplasm (Ficq, 1959). Specifically, the strands may represent a means
whereby chromosomally elaborated RNA is in the process of being transmitted into the cytoplasm. This and other material would become involved in the synthesis of proteins that is occurring in such large quantity within the cytoplasm of developing amphibian oocytes. That the
nucleoli are important centers of formation of transfer-RNA was suggested by the autoradiographic studies reported by Ozban et al. (1964).
A three-dimensional model of the amphibian oocyte nucleus at diplotene is shown in Fig. 25. The morphology of the various components is
based upon the studies reported in this paper.
VII. Summary
1. The ultrastructure of the nuclear envelope of the amphibian oocyte
is similar, if not identical, with that of somatic cells. It consists of a
pair of membranes separated by an intermembranous zone. The membranes come together regularly to form discontinuities. Within the
discontinuities are tubular structures whose ultrastructure remains to
be clarified.
2. Oogenesis is characterized by an unusually long diplotene stage
during the first prophase. During this interval the chromosomes develop
lateral loops. The ultrastructure of these elements has been described in
detail. Each chromosome consists of two chromatids which are tightly
coiled in the chromomeres and extend outward from there to form the
axes of the paired lateral loops. The functional significance of these
giant chromosomes is discussed.
3. Numerous nucleoli are formed during diplotene. They appear to
contain, at certain phases, two different granular elements. Their functional significance is evaluated.
Acknowledgment
The author is most grateful to Dr. M. S. Jacobs of the New York Zoological
Society for his critical reading of the manuscript of this paper.
References
Afzelius, B. A. (1955). Exptl. Cell Res. 8, 147.
Afzelius, B. A. (1962). J. Cell Biol. 19, 229.
Allfrey, V. G., and Mirsky, A. E. (1962). Proc. Natl. Acad. Sei. U.S. 48, 1590.
André, J., and Roullier, C. H. (1957). Proc. 1st European Regional Con]. Electron
Microscopy, Stockholm, 1956, p. 162. Academic Press, New York.
Beams, H. W. (1964). In "Cellular membranes in development" (M. Locke, ed.),
pp. 175-219. Academic Press, New York.
Beermann, W. (1952). Chromosoma 5, 139.
Born, G. (1892). Anat. Anz. 7, 772.
SAUL W I S C H N I T Z E R
berg, 1962). Such changes apparently reflect the active participation of
the nucleolus in the physiological processes taking place in the cytoplasm (Ficq, 1959). Specifically, the strands may represent a means
whereby chromosomally elaborated RNA is in the process of being transmitted into the cytoplasm. This and other material would become involved in the synthesis of proteins that is occurring in such large quantity within the cytoplasm of developing amphibian oocytes. That the
nucleoli are important centers of formation of transfer-RNA was suggested by the autoradiographic studies reported by Ozban et al. (1964).
A three-dimensional model of the amphibian oocyte nucleus at diplotene is shown in Fig. 25. The morphology of the various components is
based upon the studies reported in this paper.
VII. Summary
1. The ultrastructure of the nuclear envelope of the amphibian oocyte
is similar, if not identical, with that of somatic cells. It consists of a
pair of membranes separated by an intermembranous zone. The membranes come together regularly to form discontinuities. Within the
discontinuities are tubular structures whose ultrastructure remains to
be clarified.
2. Oogenesis is characterized by an unusually long diplotene stage
during the first prophase. During this interval the chromosomes develop
lateral loops. The ultrastructure of these elements has been described in
detail. Each chromosome consists of two chromatids which are tightly
coiled in the chromomeres and extend outward from there to form the
axes of the paired lateral loops. The functional significance of these
giant chromosomes is discussed.
3. Numerous nucleoli are formed during diplotene. They appear to
contain, at certain phases, two different granular elements. Their functional significance is evaluated.
Acknowledgment
The author is most grateful to Dr. M. S. Jacobs of the New York Zoological
Society for his critical reading of the manuscript of this paper.
References
Afzelius, B. A. (1955). Exptl. Cell Res. 8, 147.
Afzelius, B. A. (1962). J. Cell Biol. 19, 229.
Allfrey, V. G., and Mirsky, A. E. (1962). Proc. Natl. Acad. Sei. U.S. 48, 1590.
André, J., and Roullier, C. H. (1957). Proc. 1st European Regional Con]. Electron
Microscopy, Stockholm, 1956, p. 162. Academic Press, New York.
Beams, H. W. (1964). In "Cellular membranes in development" (M. Locke, ed.),
pp. 175-219. Academic Press, New York.
Beermann, W. (1952). Chromosoma 5, 139.
Born, G. (1892). Anat. Anz. 7, 772.