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SAUL WISCHNITZER
Wischnitzer, 1957). Later, Gall (1956) was able to demonstrate the
presence of the loop axis by means of electron microscopy using the
replication technique. The axis was made evident after pepsin digestion
of the matrix material. More recently, Callan and Macgregor (1958,
1962) showed that the continuity of both the interchromomeric strands
and the loops is disrupted by DNase. This finding suggests that both
have a common chemical framework which accounts for their structural
continuity.
In conclusion, it can be seen that the aforementioned studies of the
lampbrush chromosomes by electron microscopy have served primarily
to confirm the concepts formulated from phase-contrast microscope observations. Conventional sectioning techniques are of limited value in the
study of nuclei, and chromosomal material is easily damaged when prepared for replication. Technical advances however, can be expected to
improve the reliability of electron-microscope investigations (see Gall,
1963a).
V. Nucleoli
Numerous spherical bodies, which may number up to a thousand,
found within the prophase amphibian nucleus are collectively known as
nucleoli (Fig. 19A, B, and C). Our knowledge of these structures, as
for the lampbrush chromosomes, is derived by the use of microscopic as
well as microchemical techniques.
The fact that nucleoli are constant structural elements of oocyte nuclei
was originally reported by Wagner (1835). Flemming (1882) claimed
that the mature ovum had a single major nucleolus and numerous accessory nucleoli.
The changes in the intranuclear distribution of the nucleolus during
prophase has already been described (see Section IV). It was seen that
nucleolar proliferation parallels the formation of the lateral loops by the
chromosomes. Throughout the greater part of the diplotene stage they
are primarily located adjacent to the nuclear envelope. Toward the end
of this stage, they are present primarily in association with the contracting central mass of chromosomes.
The first observations of amphibian nucleoli by means of electron
microscopy in sectioned preparations, were made by Kemp (1956). He
noted that they lacked a limiting membrane, and that they appeared as
if some of their material was being shed into the surrounding nuclear
sap. These observations were subsequently confirmed by Wischnitzer
(1958) who found the nucleoli to have a homogeneous granular component (Fig. 20A,B).
An electron-microscope study using both sectioned and replicated material was carried out by Brown and Ris (1959). They found that nucleoli
SAUL WISCHNITZER
Wischnitzer, 1957). Later, Gall (1956) was able to demonstrate the
presence of the loop axis by means of electron microscopy using the
replication technique. The axis was made evident after pepsin digestion
of the matrix material. More recently, Callan and Macgregor (1958,
1962) showed that the continuity of both the interchromomeric strands
and the loops is disrupted by DNase. This finding suggests that both
have a common chemical framework which accounts for their structural
continuity.
In conclusion, it can be seen that the aforementioned studies of the
lampbrush chromosomes by electron microscopy have served primarily
to confirm the concepts formulated from phase-contrast microscope observations. Conventional sectioning techniques are of limited value in the
study of nuclei, and chromosomal material is easily damaged when prepared for replication. Technical advances however, can be expected to
improve the reliability of electron-microscope investigations (see Gall,
1963a).
V. Nucleoli
Numerous spherical bodies, which may number up to a thousand,
found within the prophase amphibian nucleus are collectively known as
nucleoli (Fig. 19A, B, and C). Our knowledge of these structures, as
for the lampbrush chromosomes, is derived by the use of microscopic as
well as microchemical techniques.
The fact that nucleoli are constant structural elements of oocyte nuclei
was originally reported by Wagner (1835). Flemming (1882) claimed
that the mature ovum had a single major nucleolus and numerous accessory nucleoli.
The changes in the intranuclear distribution of the nucleolus during
prophase has already been described (see Section IV). It was seen that
nucleolar proliferation parallels the formation of the lateral loops by the
chromosomes. Throughout the greater part of the diplotene stage they
are primarily located adjacent to the nuclear envelope. Toward the end
of this stage, they are present primarily in association with the contracting central mass of chromosomes.
The first observations of amphibian nucleoli by means of electron
microscopy in sectioned preparations, were made by Kemp (1956). He
noted that they lacked a limiting membrane, and that they appeared as
if some of their material was being shed into the surrounding nuclear
sap. These observations were subsequently confirmed by Wischnitzer
(1958) who found the nucleoli to have a homogeneous granular component (Fig. 20A,B).
An electron-microscope study using both sectioned and replicated material was carried out by Brown and Ris (1959). They found that nucleoli