226
RUTH BELLAIRS
if radioactive iron is injected into laying hens (Halkett et al., 1958).
These ingenious techniques also throw light on the way in which the
yolk is laid down in development (see Section III, B).
In the centre of the yolk is a whitish region, the latebra, which is
easily visible even in a hardboiled egg. The nucleus of Pander (see Fig.
1) is a white spot of yolk that lies beneath the surface of the vitelline
membrane of a fresh egg. In an unfertilized egg it is about 2 mm in
diameter, but in a fertilized egg it appears larger. The blastoderm lies
above it. According to Khanykova (1958), if yolks are subjected to low
frequency electrical fields, the different layers of the yolk have different
polarizabilities, the highest being in the nucleus of Pander and in the
latebra.
The bands of the yellow yolk are broader than those of the white
(about 2 mm compared with 0-25-0-4 mm according to Romanoff and
Romanoff, 1949). Thus, there is more yellow yolk than white. This may
be because the yellow bands are laid down during the day whereas the
white ones are formed at night (Riddle, 1911 ; Warren and Conrad, 1939)
when the concentration of serum proteins may be less (see Section
III, B). The amount of white yolk deposited is reduced if the bird is fed
at short intervals, and it is also affected by the type of food. That is, if
the hen routinely eats food that is rich in pigment, such as maize, there
is very little white in the yolks (Bohren et al., 1945). An additional
factor, however, is that individual hens fed on the same diet often differ
from one another in their ability to utilize the available pigments
(Romanoff and Romanoff, 1949).
Unfortunately, no satisfactory technique has yet been devised for
separating the yellow and white yolk completely. The two types of yolk
have thus been treated as if they were identical in the chemical analyses
described above.
Another way in which it can be seen that yolk is not homogeneous is
by examining a smear of yolk under the light microscope. It then appears
as a fluid with two types of structure floating in it (Fig. 5). The larger
type are usually known to embryologists as yolk spheres, but are called
globules by most yolk biochemists. The smaller are given the name of
lipid or lipoprotein drops (Fig. 7) by many embryologists, but are often
described as granules by biochemists. The yolk is generally considered
to be an emulsion of the oil-water type (Grodzinski, 1946), the continuous
phase being an aqueous protein fluid, the dispersed phase consisting of
the yolk spheres and lipoprotein drops.
The yolk spheres themselves have a number of subdroplets within
them, floating in a fluid, the yolk sphere fluid (Fig. 5). The subdroplets
are themselves lipoprotein drops. Those within the yolk spheres of the
white yolk (white yolk spheres) are highly refractile, whereas those of
RUTH BELLAIRS
if radioactive iron is injected into laying hens (Halkett et al., 1958).
These ingenious techniques also throw light on the way in which the
yolk is laid down in development (see Section III, B).
In the centre of the yolk is a whitish region, the latebra, which is
easily visible even in a hardboiled egg. The nucleus of Pander (see Fig.
1) is a white spot of yolk that lies beneath the surface of the vitelline
membrane of a fresh egg. In an unfertilized egg it is about 2 mm in
diameter, but in a fertilized egg it appears larger. The blastoderm lies
above it. According to Khanykova (1958), if yolks are subjected to low
frequency electrical fields, the different layers of the yolk have different
polarizabilities, the highest being in the nucleus of Pander and in the
latebra.
The bands of the yellow yolk are broader than those of the white
(about 2 mm compared with 0-25-0-4 mm according to Romanoff and
Romanoff, 1949). Thus, there is more yellow yolk than white. This may
be because the yellow bands are laid down during the day whereas the
white ones are formed at night (Riddle, 1911 ; Warren and Conrad, 1939)
when the concentration of serum proteins may be less (see Section
III, B). The amount of white yolk deposited is reduced if the bird is fed
at short intervals, and it is also affected by the type of food. That is, if
the hen routinely eats food that is rich in pigment, such as maize, there
is very little white in the yolks (Bohren et al., 1945). An additional
factor, however, is that individual hens fed on the same diet often differ
from one another in their ability to utilize the available pigments
(Romanoff and Romanoff, 1949).
Unfortunately, no satisfactory technique has yet been devised for
separating the yellow and white yolk completely. The two types of yolk
have thus been treated as if they were identical in the chemical analyses
described above.
Another way in which it can be seen that yolk is not homogeneous is
by examining a smear of yolk under the light microscope. It then appears
as a fluid with two types of structure floating in it (Fig. 5). The larger
type are usually known to embryologists as yolk spheres, but are called
globules by most yolk biochemists. The smaller are given the name of
lipid or lipoprotein drops (Fig. 7) by many embryologists, but are often
described as granules by biochemists. The yolk is generally considered
to be an emulsion of the oil-water type (Grodzinski, 1946), the continuous
phase being an aqueous protein fluid, the dispersed phase consisting of
the yolk spheres and lipoprotein drops.
The yolk spheres themselves have a number of subdroplets within
them, floating in a fluid, the yolk sphere fluid (Fig. 5). The subdroplets
are themselves lipoprotein drops. Those within the yolk spheres of the
white yolk (white yolk spheres) are highly refractile, whereas those of