222
RUTH BELLAIRS
characterized by a regular banding of the fibres (see Harkness, 1961)
and banding of this type was not seen in either layer of the vitelline
membrane. This finding was confirmed by a chemical analysis in which
it was found that hydroxyproline was not present. The inner and outer
layers were dissected apart and analysed separately (see Bellairs et al.,
1963). Both layers are composed mainly of protein, the amino acid
composition of which differs considerably from both collagen and
elastin and to a lesser extent from keratin. The inner layer was found to
resemble most closely certain protein fractions present in connective
tissue, whilst the outer resembled some of the proteins of egg-white. In
addition each layer contained a large amount of carbohydrate.
B. The Chemistry of the Yolk
The yolk of the laid egg has been analysed by many workers, and
much information is available in the following reviews: Marza and
Marza (1935), Needham (1950), Romanoff and Romanoff (1949), and
Warner (1954).
The average wet weight of the yolk of a domestic fowl's egg may be
taken as 18-7 g (Romanoff and Romanoff, 1949); 48-7% of this consists
of water, 16% of protein, and 33% of lipids. Similar proportions of these
components are present in the yolk of other birds (see Romanoff and
Romanoff, 1949). Needham (1931) pointed out that, compared with
other animals, birds have a particularly high fat content in their eggs.
1. Yolk Proteins
It is generally stated that the two main yolk proteins are vitellin and
livetin. Vitellin is precipitated by diluting the yolk with sodium chloride,
extracting the free lipids and dialysing the remainder. The supernatant
from the precipitate contains livetin which is water soluble (Kay and
Marshall, 1928). The properties of both these compounds have been
extensively investigated (see below). In recent years, however, it has
become apparent that these substances may not only be denatured
during preparation (see Warner, 1954) but are themselves each composed of a number of proteins. Some confusion arises from the fact that
the supposed vitellin whose properties were described by the earlier
workers now appears to be a mixture of a- and ß-vitellin, together with
vitellenin and phosvitin, although the differences between the a and ß
forms appear to be small (Cook et al., 1962).
At least six (Cook, 1961) or possibly eight (Schjeide et al., 1963c) proteins have now been extracted from yolk. Unfortunately, it has not been
easy for various authors to agree on terminology (see discussions by
Cook, 1961 ; Schjeide and Urist, 1961) or on the results that have been
RUTH BELLAIRS
characterized by a regular banding of the fibres (see Harkness, 1961)
and banding of this type was not seen in either layer of the vitelline
membrane. This finding was confirmed by a chemical analysis in which
it was found that hydroxyproline was not present. The inner and outer
layers were dissected apart and analysed separately (see Bellairs et al.,
1963). Both layers are composed mainly of protein, the amino acid
composition of which differs considerably from both collagen and
elastin and to a lesser extent from keratin. The inner layer was found to
resemble most closely certain protein fractions present in connective
tissue, whilst the outer resembled some of the proteins of egg-white. In
addition each layer contained a large amount of carbohydrate.
B. The Chemistry of the Yolk
The yolk of the laid egg has been analysed by many workers, and
much information is available in the following reviews: Marza and
Marza (1935), Needham (1950), Romanoff and Romanoff (1949), and
Warner (1954).
The average wet weight of the yolk of a domestic fowl's egg may be
taken as 18-7 g (Romanoff and Romanoff, 1949); 48-7% of this consists
of water, 16% of protein, and 33% of lipids. Similar proportions of these
components are present in the yolk of other birds (see Romanoff and
Romanoff, 1949). Needham (1931) pointed out that, compared with
other animals, birds have a particularly high fat content in their eggs.
1. Yolk Proteins
It is generally stated that the two main yolk proteins are vitellin and
livetin. Vitellin is precipitated by diluting the yolk with sodium chloride,
extracting the free lipids and dialysing the remainder. The supernatant
from the precipitate contains livetin which is water soluble (Kay and
Marshall, 1928). The properties of both these compounds have been
extensively investigated (see below). In recent years, however, it has
become apparent that these substances may not only be denatured
during preparation (see Warner, 1954) but are themselves each composed of a number of proteins. Some confusion arises from the fact that
the supposed vitellin whose properties were described by the earlier
workers now appears to be a mixture of a- and ß-vitellin, together with
vitellenin and phosvitin, although the differences between the a and ß
forms appear to be small (Cook et al., 1962).
At least six (Cook, 1961) or possibly eight (Schjeide et al., 1963c) proteins have now been extracted from yolk. Unfortunately, it has not been
easy for various authors to agree on terminology (see discussions by
Cook, 1961 ; Schjeide and Urist, 1961) or on the results that have been