YOLK OF THE H E N ' S EGG
225
3. Other Components of Yolk
Numerous other components are present in small amounts in the
yolk. These include carbohydrates, minerals and yolk pigments,
especially lipochromes and lyochromes (see Romanoff and Romanoff,
1949). Traces of egg-white proteins have also been claimed (Marshall
and Deutsch, 1951), though these have recently been re-interpreted as
transferrin (Williams, 1962b). Both RNA and DNA may be present
(Solomon, 1957) and vitamin A has also been reported (Pennock et al.,
1962).
4. Chemical Comparison with other Types of Yolk
The yolk of no other group of animals has received so much attention
as that of birds. This is partially because the importance of the fowl's
egg from a commercial point of view has stimulated its investigation,
and partially because many other yolks, such as that of the frog's egg,
are small and contaminated by cytoplasm. However, lipovitellins have
been found not only in the yolk of amphibians (Fujii, 1960; Radomski
et al., 1963; Wallace, 1963) but also in that of other vertebrate species
such as Salmo irridaeus and Scylliorhinus stellenius (Fujii, 1960). The
lipovitellin of Ranapipiens resembles avian a-lipovitellin in its properties,
but there is nothing in this animal which corresponds to avian
/Mipovitellin (Radomski et al., 1963). A phosphoprotein similar to
phosvitin is also found in amphibian yolk (Wallace, 1963).
C. The Physical Structure of the Yolk
Chemical investigations of the type described above provide us with
a great deal of information otherwise unobtainable, but they have one
major drawback for the biologist. They have until recently involved
treating yolk as if it were morphologically the same throughout.
It has been known for many years that the yolk of the laid egg is not
a homogeneous mass. Thompson (1859), who probably first described
the stratification of the yolk into yellow and white layers, demonstrated
it by hardboiling an egg and then slicing it down the middle. Using this
method, however, the stratification is not always distinct and some
authors (see reviews of literature by Marza and Marza, 1935) have
denied that it exists; others have objected that whilst a stratification
does occur it is frequently of various shades of yellow and that it is not
necessarily concentric. The stratification can, however, be demonstrated
in another way; i.e. if fat-soluble dyes, such as Sudan III, are fed to
(Riddle, 1911) or injected into (Warren and Conrad, 1939) a laying hen
at intervals, they can subsequently be found arranged in more or less
concentric bands in the yolk of the laid egg. A similar result is obtained
225
3. Other Components of Yolk
Numerous other components are present in small amounts in the
yolk. These include carbohydrates, minerals and yolk pigments,
especially lipochromes and lyochromes (see Romanoff and Romanoff,
1949). Traces of egg-white proteins have also been claimed (Marshall
and Deutsch, 1951), though these have recently been re-interpreted as
transferrin (Williams, 1962b). Both RNA and DNA may be present
(Solomon, 1957) and vitamin A has also been reported (Pennock et al.,
1962).
4. Chemical Comparison with other Types of Yolk
The yolk of no other group of animals has received so much attention
as that of birds. This is partially because the importance of the fowl's
egg from a commercial point of view has stimulated its investigation,
and partially because many other yolks, such as that of the frog's egg,
are small and contaminated by cytoplasm. However, lipovitellins have
been found not only in the yolk of amphibians (Fujii, 1960; Radomski
et al., 1963; Wallace, 1963) but also in that of other vertebrate species
such as Salmo irridaeus and Scylliorhinus stellenius (Fujii, 1960). The
lipovitellin of Ranapipiens resembles avian a-lipovitellin in its properties,
but there is nothing in this animal which corresponds to avian
/Mipovitellin (Radomski et al., 1963). A phosphoprotein similar to
phosvitin is also found in amphibian yolk (Wallace, 1963).
C. The Physical Structure of the Yolk
Chemical investigations of the type described above provide us with
a great deal of information otherwise unobtainable, but they have one
major drawback for the biologist. They have until recently involved
treating yolk as if it were morphologically the same throughout.
It has been known for many years that the yolk of the laid egg is not
a homogeneous mass. Thompson (1859), who probably first described
the stratification of the yolk into yellow and white layers, demonstrated
it by hardboiling an egg and then slicing it down the middle. Using this
method, however, the stratification is not always distinct and some
authors (see reviews of literature by Marza and Marza, 1935) have
denied that it exists; others have objected that whilst a stratification
does occur it is frequently of various shades of yellow and that it is not
necessarily concentric. The stratification can, however, be demonstrated
in another way; i.e. if fat-soluble dyes, such as Sudan III, are fed to
(Riddle, 1911) or injected into (Warren and Conrad, 1939) a laying hen
at intervals, they can subsequently be found arranged in more or less
concentric bands in the yolk of the laid egg. A similar result is obtained