YOLK OF THE H E N ' S EGG
263
yolk sac. Lipases, however, do not reach their maximum activity until
about 16 days (Needham, 1950; Riddle, 1916). After this stage the
proportion of unesterified fatty acid in the yolk sac lipid decreases,
whilst the proportion of free and esterified cholesterol increases (Moore
and Doran, 1962). Meanwhile, the lecithin phosphorus, cephalin phosphorus, total lipid choline and free cholesterol gradually decrease in the
yolk during incubation and increase in the embryo at the same time
(TsujiefaZ., 1955).
V I I . Development of the Embryo without Yolk
The tissues of the bird embryo are formed from yolk and egg-white
that are broken down and absorbed. I t seems reasonable to assume,
therefore, that these substances are particularly suited to the task that
they have to perform.
Many attempts have been made to replace the yolk by alternative
raw materials, either by removing the young blastoderm from the egg
and explanting it in vitro, or by leaving the young embryo in situ and
substituting nutrient media for yolk. I t is perhaps significant that in
both types of experiment the greatest successes have been obtained
where either yolk or egg-white have been included in the medium. Both
techniques are useful, however, as a means for investigating the uptake
of new materials by the embryo.
A. The Explantation Technique
This technique was introduced for chick embryos by Waddington
(1932) who placed the blastoderm on a firm clot made from fowl's blood
plasma, saline and embryo extract. This was a landmark in experimental
embryology as it enabled many grafting experiments to be carried out
that had hitherto been impossible. The embryo can survive for 1-2 days
and good differentiation occurs, though a well-developed extraembryonic circulation is seldom achieved. Other authors have modified
the technique so that this difficulty has been overcome. Excellent results
have been obtained by stretching the vitelline membrane with the
blastoderm still attached to it over egg-white (New, 1955; Gallera and
Nicolet, 1961). The egg-white apparently supplies the nutrients required at this time (see below) and the vitelline membrane provides the
correct substrate over which the blastoderm can migrate (see Section
V,A,2).
Spratt (1948) has modified Waddington's technique by replacing the
plasma clot with various alternative media, the best being a mixture of
yolk and albumen. This enables differentiation to proceed, but expansion of the blastoderm is not so satisfactory as it is with New's
13
A . M . 4
263
yolk sac. Lipases, however, do not reach their maximum activity until
about 16 days (Needham, 1950; Riddle, 1916). After this stage the
proportion of unesterified fatty acid in the yolk sac lipid decreases,
whilst the proportion of free and esterified cholesterol increases (Moore
and Doran, 1962). Meanwhile, the lecithin phosphorus, cephalin phosphorus, total lipid choline and free cholesterol gradually decrease in the
yolk during incubation and increase in the embryo at the same time
(TsujiefaZ., 1955).
V I I . Development of the Embryo without Yolk
The tissues of the bird embryo are formed from yolk and egg-white
that are broken down and absorbed. I t seems reasonable to assume,
therefore, that these substances are particularly suited to the task that
they have to perform.
Many attempts have been made to replace the yolk by alternative
raw materials, either by removing the young blastoderm from the egg
and explanting it in vitro, or by leaving the young embryo in situ and
substituting nutrient media for yolk. I t is perhaps significant that in
both types of experiment the greatest successes have been obtained
where either yolk or egg-white have been included in the medium. Both
techniques are useful, however, as a means for investigating the uptake
of new materials by the embryo.
A. The Explantation Technique
This technique was introduced for chick embryos by Waddington
(1932) who placed the blastoderm on a firm clot made from fowl's blood
plasma, saline and embryo extract. This was a landmark in experimental
embryology as it enabled many grafting experiments to be carried out
that had hitherto been impossible. The embryo can survive for 1-2 days
and good differentiation occurs, though a well-developed extraembryonic circulation is seldom achieved. Other authors have modified
the technique so that this difficulty has been overcome. Excellent results
have been obtained by stretching the vitelline membrane with the
blastoderm still attached to it over egg-white (New, 1955; Gallera and
Nicolet, 1961). The egg-white apparently supplies the nutrients required at this time (see below) and the vitelline membrane provides the
correct substrate over which the blastoderm can migrate (see Section
V,A,2).
Spratt (1948) has modified Waddington's technique by replacing the
plasma clot with various alternative media, the best being a mixture of
yolk and albumen. This enables differentiation to proceed, but expansion of the blastoderm is not so satisfactory as it is with New's
13
A . M . 4