YOLK OF THE H E N ' S EGG
265
B. Yolk Replacement in situ
This technique enables the yolk to be flushed out of an egg that has
been incubated for 3 or more days. The yolk sac remains intact and the
cavity it encloses can subsequently be filled with other media (Grau et
al., 1957; Klein et ah, 1958). These authors found that the more yolk
that was included in the new medium the longer the embryo could
survive. With 95% yolk it was possible for hatching to take place. When
a 4-day embryo was offered nothing but glucose and saline by perfusion,
the average length of survival was only 14 h.
V I I I . Post-Hatching Fate of Yolk and Yolk Sac
The yolk sac remains attached to the midgut by the yolk sac stalk
throughout embryonic life. During the early stages of development
(about 6 days in the chick) a loop of midgut herniates into the extraembryonic coelom in much the same way as it does in mammals. In the
last week of incubation, movements take place in the abdominal wall
that not only retract the now somewhat modified gut back into the
abdomen, but also draw in the yolk sac stalk and the yolk sac as well.
The midgut loop enters the body at about 19 days of incubation, and the
yolk sac has usually been drawn in by the time of hatching on the 21st
day. Kuo (1932) has shown that the movements are brought about by
muscular activity of the abdominal wall.
The yolk sac contains a certain amount of unused yolk (residual yolk)
and this is, therefore, also carried into the abdominal cavity. According
to Virchow (1891) there are about 5-34 g of yolk at this time. Most
workers agree that the greater part of the residual yolk is used up during
the first 6 or 7 days after hatching.
The way in which this yolk is absorbed from the yolk sac has been
investigated by many workers using both histological and experimental
approaches, but the conclusions they have reached are not unanimous.
There are two possible routes. The first is by passing up the lumen of
the yolk sac stalk into the midgut of the young chick. This route would
perhaps be the more surprising one because there is no evidence that
yolk is taken up by the embryo in this way (see Section V). The alternative route is by passing through the cells of the yolk sac itself into the
circulation of the young bird; most of the evidence favours this
hypothesis.
Experiments in which the residual yolk was marked with trypan blue
or Indian ink have been interpreted as demonstrating that the transfer
does occur through the lumen (Knake, 1935), that it does not occur in
this way (Fritz, 1961) or that it does so only after the greater part has
265
B. Yolk Replacement in situ
This technique enables the yolk to be flushed out of an egg that has
been incubated for 3 or more days. The yolk sac remains intact and the
cavity it encloses can subsequently be filled with other media (Grau et
al., 1957; Klein et ah, 1958). These authors found that the more yolk
that was included in the new medium the longer the embryo could
survive. With 95% yolk it was possible for hatching to take place. When
a 4-day embryo was offered nothing but glucose and saline by perfusion,
the average length of survival was only 14 h.
V I I I . Post-Hatching Fate of Yolk and Yolk Sac
The yolk sac remains attached to the midgut by the yolk sac stalk
throughout embryonic life. During the early stages of development
(about 6 days in the chick) a loop of midgut herniates into the extraembryonic coelom in much the same way as it does in mammals. In the
last week of incubation, movements take place in the abdominal wall
that not only retract the now somewhat modified gut back into the
abdomen, but also draw in the yolk sac stalk and the yolk sac as well.
The midgut loop enters the body at about 19 days of incubation, and the
yolk sac has usually been drawn in by the time of hatching on the 21st
day. Kuo (1932) has shown that the movements are brought about by
muscular activity of the abdominal wall.
The yolk sac contains a certain amount of unused yolk (residual yolk)
and this is, therefore, also carried into the abdominal cavity. According
to Virchow (1891) there are about 5-34 g of yolk at this time. Most
workers agree that the greater part of the residual yolk is used up during
the first 6 or 7 days after hatching.
The way in which this yolk is absorbed from the yolk sac has been
investigated by many workers using both histological and experimental
approaches, but the conclusions they have reached are not unanimous.
There are two possible routes. The first is by passing up the lumen of
the yolk sac stalk into the midgut of the young chick. This route would
perhaps be the more surprising one because there is no evidence that
yolk is taken up by the embryo in this way (see Section V). The alternative route is by passing through the cells of the yolk sac itself into the
circulation of the young bird; most of the evidence favours this
hypothesis.
Experiments in which the residual yolk was marked with trypan blue
or Indian ink have been interpreted as demonstrating that the transfer
does occur through the lumen (Knake, 1935), that it does not occur in
this way (Fritz, 1961) or that it does so only after the greater part has