Y O L K O F T H E H E N ' S E G G
247
In describing the complex yolk drops in 1958, I reported that mitochondria were sometimes found within them, and advanced several
possible explanations for their presence, one being that they were
mitochondria that had been there from the time of yolk formation
during oogenesis. Subsequently, however, I discovered that some of the
intracellular structures that I had identified as yolk were phagocytosed
pieces of degenerating cells| (Bellairs, 1961a) and that these were the
structures that contained the mitochondria. The intimate association
between yolk and mitochondria, now so well established for amphibian
embryos (Sung, 1961 ; Ward, 1962; Karasaki, 1963), does not appear to
hold for the chick, at least in the embryonic stages.
Investigators using light microscope techniques have also reported
that yolk drops may appear different in different parts of the blastoderm
and that they change their appearance as they become digested (Konopacka, 1933; Thomas, 1938; Grodzinski, 1946). It seems to be generally
accepted that the intracellular yolk drops consist essentially of lipids
and proteins. The lipids are easily demonstrated: for instance, by staining with Sudan I I I (Thomas, 1938; Grodzinski, 1946; ByczowskaSmyk, 1956), Sudan black B (Romanini, 1960) or osmium tetroxide
(Thomas, 1938; Grodzinski, 1946). Some authors also claim to have
identified specific lipids.
By contrast, the protein component of the intracellular yolk drops is
less easily demonstrated and not all investigators have agreed as to its
nature. None of the evidence on which their conclusions are based
seems to be completely convincing, since the techniques available have
not been entirely specific. For instance, dyes such as Nile blue sulphate
and neutral red have been used, and although all the authors have used
a variety of staining techniques in their investigations, few have carried
out a parallel series of investigations on control material. The results
from the use of proteolytic enzymes have been disappointing as they
have tended to cause a coagulation of the yolk substance (Grodzinski,
1946). Two main views on the nature of the protein component have,
however, been put forward. The first is that the intracellular yolk
consists of an aqueous protein fluid in which float quite separate lipid
drops (Grodzinski, 1946). This view implies that the intracellular yolk
drops differ from the extracellular (extra-embryonic) yolk spheres only
in that they are enclosed within the cell. The second view is that the
•j· Degenerating cells are a common feature of normal, healthy embryos (see Glücksmann, 1951). They resemble the complex drops in being more electron dense than the
neighbouring healthy cells. They are usually surrounded by two unit membranes, however, whereas the complex drops are surrounded by but one (see below). The degenerating
cells sometimes possess very large granules arranged in a regular pattern of rows, the
details of this pattern varying from one cell to another, presumably according to the
state of autolysis and to variations in the breakdown process.
247
In describing the complex yolk drops in 1958, I reported that mitochondria were sometimes found within them, and advanced several
possible explanations for their presence, one being that they were
mitochondria that had been there from the time of yolk formation
during oogenesis. Subsequently, however, I discovered that some of the
intracellular structures that I had identified as yolk were phagocytosed
pieces of degenerating cells| (Bellairs, 1961a) and that these were the
structures that contained the mitochondria. The intimate association
between yolk and mitochondria, now so well established for amphibian
embryos (Sung, 1961 ; Ward, 1962; Karasaki, 1963), does not appear to
hold for the chick, at least in the embryonic stages.
Investigators using light microscope techniques have also reported
that yolk drops may appear different in different parts of the blastoderm
and that they change their appearance as they become digested (Konopacka, 1933; Thomas, 1938; Grodzinski, 1946). It seems to be generally
accepted that the intracellular yolk drops consist essentially of lipids
and proteins. The lipids are easily demonstrated: for instance, by staining with Sudan I I I (Thomas, 1938; Grodzinski, 1946; ByczowskaSmyk, 1956), Sudan black B (Romanini, 1960) or osmium tetroxide
(Thomas, 1938; Grodzinski, 1946). Some authors also claim to have
identified specific lipids.
By contrast, the protein component of the intracellular yolk drops is
less easily demonstrated and not all investigators have agreed as to its
nature. None of the evidence on which their conclusions are based
seems to be completely convincing, since the techniques available have
not been entirely specific. For instance, dyes such as Nile blue sulphate
and neutral red have been used, and although all the authors have used
a variety of staining techniques in their investigations, few have carried
out a parallel series of investigations on control material. The results
from the use of proteolytic enzymes have been disappointing as they
have tended to cause a coagulation of the yolk substance (Grodzinski,
1946). Two main views on the nature of the protein component have,
however, been put forward. The first is that the intracellular yolk
consists of an aqueous protein fluid in which float quite separate lipid
drops (Grodzinski, 1946). This view implies that the intracellular yolk
drops differ from the extracellular (extra-embryonic) yolk spheres only
in that they are enclosed within the cell. The second view is that the
•j· Degenerating cells are a common feature of normal, healthy embryos (see Glücksmann, 1951). They resemble the complex drops in being more electron dense than the
neighbouring healthy cells. They are usually surrounded by two unit membranes, however, whereas the complex drops are surrounded by but one (see below). The degenerating
cells sometimes possess very large granules arranged in a regular pattern of rows, the
details of this pattern varying from one cell to another, presumably according to the
state of autolysis and to variations in the breakdown process.