YOLK OF THE H E N ' S EGG
245
cells of the area vitellina growing in tissue culture. She found that within
a few minutes the intracellular yolk drops burst, presumably because a
large amount of water passed through each yolk drop membrane by
osmosis. If similar cells were treated with hypertonic solutions (Datkowna, 1949b) the lipid drops within the yolk drops fused together and
the yolk drops membrane became wrinkled.
The structural appearance of the intracellular yolk varies even in the
same cell. In 1958 I distinguished three types of yolk in the area pellucida which I called type A, type B, and complex.
Type A is characterized by a dense core surrounded by a less dense
region, although occasionally the whole yolk drop may appear uniformly granular throughout. Type A drops are oval or round in section
(Fig. 18) and are usually between about 0-5 and 4 μ in diameter in the
area pellucida. They are surrounded by a single unit membrane. They
are thought to be composed of lipoproteins, but it is important to note
that their structure is not identical with that of the extracellular yolk.
Type B are usually electron dense after fixation in osmium tetroxide
and they are homogeneous. They are not fixed by potassium permanganate and are apparently dissolved out of the material during preparation since they are then visible only as holes in the sections (Bellairs,
1963). Unlike type A and complex drops they are not surrounded by
unit membranes and they do not resemble any of the usual components
of unincubated yolk (see Bellairs, 1961b), but are more like the lipid drops
commonly found in other tissues. Although it seems likely that the type
B drops are composed of lipids derived from yolk there is no direct
evidence that this is so. It is perhaps of significance, however, that
structures closely resembling the type B drops appear to be formed in
the extra-embryonic yolk during incubation (preliminary observation
by Bellairs, 1961b).
The complex yolk drops are perhaps best thought of as immense type
A drops whose components are arranged in a less well-ordered manner
(Fig. 19). They are not common in the area pellucida but constitute almost the entire population of yolk drops in the area opaca with the
exception of a few type B drops. In the area opaca the complex drops
may reach up to 80 μ in diameter. They not only contain the same
structures that are found in the type A drops but also membranes and
Vacuoles' and sometimes type B drops. I t is now believed (Bellairs,
1963) that these membranes and vacuoles have been caused by the
hydration of the yolk just as similar structures are believed to have
formed in the periblast (see Section V, A, 1). Unfortunately, there is no
evidence to indicate whether a hydration of this type occurred to the
already intracellular yolk or whether it took place in extracellular yolk
which subsequently became phagocytosed.
245
cells of the area vitellina growing in tissue culture. She found that within
a few minutes the intracellular yolk drops burst, presumably because a
large amount of water passed through each yolk drop membrane by
osmosis. If similar cells were treated with hypertonic solutions (Datkowna, 1949b) the lipid drops within the yolk drops fused together and
the yolk drops membrane became wrinkled.
The structural appearance of the intracellular yolk varies even in the
same cell. In 1958 I distinguished three types of yolk in the area pellucida which I called type A, type B, and complex.
Type A is characterized by a dense core surrounded by a less dense
region, although occasionally the whole yolk drop may appear uniformly granular throughout. Type A drops are oval or round in section
(Fig. 18) and are usually between about 0-5 and 4 μ in diameter in the
area pellucida. They are surrounded by a single unit membrane. They
are thought to be composed of lipoproteins, but it is important to note
that their structure is not identical with that of the extracellular yolk.
Type B are usually electron dense after fixation in osmium tetroxide
and they are homogeneous. They are not fixed by potassium permanganate and are apparently dissolved out of the material during preparation since they are then visible only as holes in the sections (Bellairs,
1963). Unlike type A and complex drops they are not surrounded by
unit membranes and they do not resemble any of the usual components
of unincubated yolk (see Bellairs, 1961b), but are more like the lipid drops
commonly found in other tissues. Although it seems likely that the type
B drops are composed of lipids derived from yolk there is no direct
evidence that this is so. It is perhaps of significance, however, that
structures closely resembling the type B drops appear to be formed in
the extra-embryonic yolk during incubation (preliminary observation
by Bellairs, 1961b).
The complex yolk drops are perhaps best thought of as immense type
A drops whose components are arranged in a less well-ordered manner
(Fig. 19). They are not common in the area pellucida but constitute almost the entire population of yolk drops in the area opaca with the
exception of a few type B drops. In the area opaca the complex drops
may reach up to 80 μ in diameter. They not only contain the same
structures that are found in the type A drops but also membranes and
Vacuoles' and sometimes type B drops. I t is now believed (Bellairs,
1963) that these membranes and vacuoles have been caused by the
hydration of the yolk just as similar structures are believed to have
formed in the periblast (see Section V, A, 1). Unfortunately, there is no
evidence to indicate whether a hydration of this type occurred to the
already intracellular yolk or whether it took place in extracellular yolk
which subsequently became phagocytosed.