228
RUTH BELLAIRS
the yellow yolk spheres are not. The subdroplets of the white yolk
spheres are also larger and less numerous than those in the yellow yolk
spheres. The number of subdroplets is, however, very variable, white
yolk spheres possessing any number from, say, one to fifty or more.
The white yolk spheres vary in diameter from about 4 μ to about
75 μ, whereas the yellow ones are between about 25 μ and 150 μ
(Romanoff and Romanoff, 1949). The lipoprotein drops tend to be about
2-10/x.
The concept that the white and yellow yolk spheres differed from one
another was introduced by Schwann (1847) but many authors have not
distinguished between them (see discussion by Marza and Marza, 1935).
Others (e.g. Riddle, 1911) have suggested that white yolk spheres are
merely a stage in the formation of the yellow yolk spheres.
When yolk is examined by electron microscopy at low magnifications,
the structures visible by light microscopy can be recognized and it is
possible to distinguish between yellow and white yolk spheres. At high
electron microscope magnifications, however, only three types of structure are generally visible: membranes (discussed below), particles with
a diameter of 30-60 Â (Fig. 8) and profiles with a diameter of about
250 Â (see Fig. 6). The small particles are the only visible component of
the subdroplets of both yellow and white yolk and of the free-floating
lipoprotein drops. It thus seems possible that both types of subdroplets
and the free-floating drops are similar in constitution, although it must
be admitted that different structures may look the same under the
electron microscope.
The 250 Â profiles are found in the continuous phase fluid and in the
fluid of each yolk sphere ; these are generally said to be aqueous protein
fluids. They are often arranged together to form rings or chains, though
it is not known whether this is a preparation artifact. Again, it seems
possible that the two types of protein fluid are identical.
Until very recently, our only knowledge about the chemical nature of
the individual yolk components was derived from histochemical studies.
For instance, Grodzinski (1939, 1946) treated yolk with Sudan I I I and
found that the subdroplets of both yellow and white yolk spheres
consisted at least in part of lipids. Similarly, Konopacka (1933) and
Thomas (1938) as a result of histochemical investigations concluded
that the continuous phase fluid was an aqueous protein fluid, whereas
the subdroplets were lipoproteins. Improved techniques of ultracentrifugation now make it possible to separate the free-floating lipoprotein drops (known as granules by the biochemists) from the rest of
the yolk. The 'granules' have been found to contain 23% of the yolk
solids (Burley and Cook, 1961) largely in the form of lipovitellins
(Schjeide and Urist, 1959; Hasegawa, 1960a; Burley and Cook, 1961;
RUTH BELLAIRS
the yellow yolk spheres are not. The subdroplets of the white yolk
spheres are also larger and less numerous than those in the yellow yolk
spheres. The number of subdroplets is, however, very variable, white
yolk spheres possessing any number from, say, one to fifty or more.
The white yolk spheres vary in diameter from about 4 μ to about
75 μ, whereas the yellow ones are between about 25 μ and 150 μ
(Romanoff and Romanoff, 1949). The lipoprotein drops tend to be about
2-10/x.
The concept that the white and yellow yolk spheres differed from one
another was introduced by Schwann (1847) but many authors have not
distinguished between them (see discussion by Marza and Marza, 1935).
Others (e.g. Riddle, 1911) have suggested that white yolk spheres are
merely a stage in the formation of the yellow yolk spheres.
When yolk is examined by electron microscopy at low magnifications,
the structures visible by light microscopy can be recognized and it is
possible to distinguish between yellow and white yolk spheres. At high
electron microscope magnifications, however, only three types of structure are generally visible: membranes (discussed below), particles with
a diameter of 30-60 Â (Fig. 8) and profiles with a diameter of about
250 Â (see Fig. 6). The small particles are the only visible component of
the subdroplets of both yellow and white yolk and of the free-floating
lipoprotein drops. It thus seems possible that both types of subdroplets
and the free-floating drops are similar in constitution, although it must
be admitted that different structures may look the same under the
electron microscope.
The 250 Â profiles are found in the continuous phase fluid and in the
fluid of each yolk sphere ; these are generally said to be aqueous protein
fluids. They are often arranged together to form rings or chains, though
it is not known whether this is a preparation artifact. Again, it seems
possible that the two types of protein fluid are identical.
Until very recently, our only knowledge about the chemical nature of
the individual yolk components was derived from histochemical studies.
For instance, Grodzinski (1939, 1946) treated yolk with Sudan I I I and
found that the subdroplets of both yellow and white yolk spheres
consisted at least in part of lipids. Similarly, Konopacka (1933) and
Thomas (1938) as a result of histochemical investigations concluded
that the continuous phase fluid was an aqueous protein fluid, whereas
the subdroplets were lipoproteins. Improved techniques of ultracentrifugation now make it possible to separate the free-floating lipoprotein drops (known as granules by the biochemists) from the rest of
the yolk. The 'granules' have been found to contain 23% of the yolk
solids (Burley and Cook, 1961) largely in the form of lipovitellins
(Schjeide and Urist, 1959; Hasegawa, 1960a; Burley and Cook, 1961;