YOLK OF THE HEN'S EGG
249
time for its properties change, not only as judged by its staining reactions, but also as seen in the polarizing microscope. Having been
isotropic it now becomes anisotropic (Grodzinski, 1946; Thomas, 1938;
Byczowska-Smyk, 1956) and it is generally considered that it turns into
glycerides (Grodzinski, 1946; Thomas, 1938). Konopacka (1933) has
suggested that lecithin becomes detached from the lipoprotein at the
time when the lipid drops become conspicuous. It is possible that the
type B drops which have been seen by electron microscopy in the large
intracellular yolk drops of both the area pellucida and area opaca, are
breakdown products of digestion, for they do not appear in the extracellular yolk until later in development (Bellairs, 1961b). They may correspond with the glycerides reported above.
Various authors have also concluded from staining reactions that
there is a hydration of the yolk in the area vitellina (Thomas, 1938;
Grodzinski, 1946). The appearance of the myelin figure-like membranes
seen in the yolk by electron microscopy suggest that this is correct (see
Section V, A, 1).
The second conclusion to be drawn about the process of yolk digestion
is that some of the large yolk drops of the area opaca become vacuolated.
This has been seen by light microscopy (Konopacka, 1933; Grodzinski,
1939; Thomas, 1938), and by electron microscopy (Bellairs, 1963).
Vacuolation of this type however, is particularly conspicuous in the large
yolk drops of the area opaca. It is apparent that vacuolation is due to
some change in the yolk, but whether this involves a passage of substances into the cytoplasm at this stage is not known.
The third main visible event in yolk digestion is that the membrane
surrounding the yolk sphere breaks down (Grodzinski, 1946; Bellairs,
1958) and the lipid drops and the lipoproteins pass out into the cytoplasm of the cell. The lipoproteins, stained red with neutral red, soon
disappear in the cell and the small lipid drops remain (Grodzinski,
1946). The vacuolated bodies seen by electron microscopy become
indistinguishable from endoplasmic reticulum (Bellairs, 1958). The
breakdown of the membrane surrounding an intracellular yolk drop
can be brought about experimentally by treatment with lipases but not
with proteases (Grodzinski, 1946; Byczowska-Smyk, 1956).
A. The Rate of Disappearance of Yolk
During the period between about 24 and 48 h of incubation, the intracellular yolk drops of the area pellucida undergo structural changes
(Bellairs, 1958) which suggests that they are being digested by the cells.
There is reason to believe that during this early period the intracellular
yolk provides almost the whole of the food supply for the area pellucida,
for if chick blastoderms are dissected off the yolk they can survive in
249
time for its properties change, not only as judged by its staining reactions, but also as seen in the polarizing microscope. Having been
isotropic it now becomes anisotropic (Grodzinski, 1946; Thomas, 1938;
Byczowska-Smyk, 1956) and it is generally considered that it turns into
glycerides (Grodzinski, 1946; Thomas, 1938). Konopacka (1933) has
suggested that lecithin becomes detached from the lipoprotein at the
time when the lipid drops become conspicuous. It is possible that the
type B drops which have been seen by electron microscopy in the large
intracellular yolk drops of both the area pellucida and area opaca, are
breakdown products of digestion, for they do not appear in the extracellular yolk until later in development (Bellairs, 1961b). They may correspond with the glycerides reported above.
Various authors have also concluded from staining reactions that
there is a hydration of the yolk in the area vitellina (Thomas, 1938;
Grodzinski, 1946). The appearance of the myelin figure-like membranes
seen in the yolk by electron microscopy suggest that this is correct (see
Section V, A, 1).
The second conclusion to be drawn about the process of yolk digestion
is that some of the large yolk drops of the area opaca become vacuolated.
This has been seen by light microscopy (Konopacka, 1933; Grodzinski,
1939; Thomas, 1938), and by electron microscopy (Bellairs, 1963).
Vacuolation of this type however, is particularly conspicuous in the large
yolk drops of the area opaca. It is apparent that vacuolation is due to
some change in the yolk, but whether this involves a passage of substances into the cytoplasm at this stage is not known.
The third main visible event in yolk digestion is that the membrane
surrounding the yolk sphere breaks down (Grodzinski, 1946; Bellairs,
1958) and the lipid drops and the lipoproteins pass out into the cytoplasm of the cell. The lipoproteins, stained red with neutral red, soon
disappear in the cell and the small lipid drops remain (Grodzinski,
1946). The vacuolated bodies seen by electron microscopy become
indistinguishable from endoplasmic reticulum (Bellairs, 1958). The
breakdown of the membrane surrounding an intracellular yolk drop
can be brought about experimentally by treatment with lipases but not
with proteases (Grodzinski, 1946; Byczowska-Smyk, 1956).
A. The Rate of Disappearance of Yolk
During the period between about 24 and 48 h of incubation, the intracellular yolk drops of the area pellucida undergo structural changes
(Bellairs, 1958) which suggests that they are being digested by the cells.
There is reason to believe that during this early period the intracellular
yolk provides almost the whole of the food supply for the area pellucida,
for if chick blastoderms are dissected off the yolk they can survive in