U L T R A S T R U C T U R E OF T H E A M P H I B I A N EGG
189
150-200 Â in diameter, whereas fibers are 40-70 Â thick and are
very coiled. Moreover, prior to yolk formation, the nucleoli are compact
and consist only of fine granules. During yolk formation a change in
nucleolar composition becomes evident and both coarse and fine components can be seen. The coarse particles are loosely organized and
serve to envelope vacuolar-like areas. These observations are in agreement with those made by phase-contrast microscopy (Wischnitzer,
unpublished).
Miller (1962) studied oocytes from Triturus viridescens and Rana
clamitans. He too, found that the majority of nucleoli exhibit two distinct morphological components: a loose granular cortex which usually
surrounds a dense fibrous core. However, he reported both structural
compounds of the nucleolus as being present during all growth stages.
VI. Discussion
Electron microscopy of the nuclear envelope of amphibian eggs has
contributed to both the discovery and establishment of the constant
structural components of this entity. The latter consist of the bilamellar
elements and the annular complex. The exact morphology of the annular
complex, however, remains to be clarified. It is tubular in shape and is
thought by some (Watson, 1959; Miller, 1962) to represent extruded
nuclear material. Most workers (Afzelius, 1955; Wischnitzer, 1958;
Merriam, 1961) feel that the cylindrical components are true structural
elements associated with the annuli. Similarly, the annular wall, central
annular granule, and the annular diaphragm are structures whose significance remains to be elucidated.
The study of the fine structure of the nuclear envelope is motivated
not only by a desire to clarify the morphology of this structure, but also
to gain an insight regarding the role, active or passive, that this physical
barrier plays in nucleocytoplasmic exchange. Based upon morphological
data obtained by electron microscopy, several theories have been offered
which postulate possible routes for such exchange. These theories will
be described briefly.
1. The central annular granules are involved in the transmission of
material. This concept is supported by experimental studies with both
amoebae (Feldherr, 1962, 1965) and isolated amphibian nuclei (Feldherr,
1963).
2. Nuclear material diffuses from the nucleus into the perinuclear
space and then into the endoplasmic reticulum (Watson, 1955).
3. Material, in the form of large masses or vesicles, is extruded
through the membrane which later "repairs" itself (Gay, 1956; Afzelius,
1962).
4. The annular lamellae are intranuclear products that pass from the
189
150-200 Â in diameter, whereas fibers are 40-70 Â thick and are
very coiled. Moreover, prior to yolk formation, the nucleoli are compact
and consist only of fine granules. During yolk formation a change in
nucleolar composition becomes evident and both coarse and fine components can be seen. The coarse particles are loosely organized and
serve to envelope vacuolar-like areas. These observations are in agreement with those made by phase-contrast microscopy (Wischnitzer,
unpublished).
Miller (1962) studied oocytes from Triturus viridescens and Rana
clamitans. He too, found that the majority of nucleoli exhibit two distinct morphological components: a loose granular cortex which usually
surrounds a dense fibrous core. However, he reported both structural
compounds of the nucleolus as being present during all growth stages.
VI. Discussion
Electron microscopy of the nuclear envelope of amphibian eggs has
contributed to both the discovery and establishment of the constant
structural components of this entity. The latter consist of the bilamellar
elements and the annular complex. The exact morphology of the annular
complex, however, remains to be clarified. It is tubular in shape and is
thought by some (Watson, 1959; Miller, 1962) to represent extruded
nuclear material. Most workers (Afzelius, 1955; Wischnitzer, 1958;
Merriam, 1961) feel that the cylindrical components are true structural
elements associated with the annuli. Similarly, the annular wall, central
annular granule, and the annular diaphragm are structures whose significance remains to be elucidated.
The study of the fine structure of the nuclear envelope is motivated
not only by a desire to clarify the morphology of this structure, but also
to gain an insight regarding the role, active or passive, that this physical
barrier plays in nucleocytoplasmic exchange. Based upon morphological
data obtained by electron microscopy, several theories have been offered
which postulate possible routes for such exchange. These theories will
be described briefly.
1. The central annular granules are involved in the transmission of
material. This concept is supported by experimental studies with both
amoebae (Feldherr, 1962, 1965) and isolated amphibian nuclei (Feldherr,
1963).
2. Nuclear material diffuses from the nucleus into the perinuclear
space and then into the endoplasmic reticulum (Watson, 1955).
3. Material, in the form of large masses or vesicles, is extruded
through the membrane which later "repairs" itself (Gay, 1956; Afzelius,
1962).
4. The annular lamellae are intranuclear products that pass from the