Y O L K OF T H E H E N ' S EGG
259
Long arms of cytoplasm were occasionally found extended around large
yolk masses as if the cells have been fixed during the process of engulfing
yolk (Bellairs, 1963).
Trypan blue, if injected beneath the yolk sac, also enters the cells but
probably by diffusion rather than phagocytosis (Wislocki, 1921; Latta
and Busby, 1929).
Soluble
32
P when injected into the yolk sac is subsequently found in
the blood and body of the embryo (Taylor and Saenz, 1949). I t seems
possible that this too is due to diffusion rather than ingestion proper,
since the
32
P is not incorporated into the yolk phospholipids (Webster
and Ward, 1953). Similarly, although it has been found that viruses or
specific antibodies are taken up by the embryo after injection into the
yolk (Brandly et al., 1946 ; Brierley and Hemmings, 1956) the mechanism
is not properly understood, though it seems clear that the vitelline circulation is involved (Buxton, 1952; Brierley and Hemmings, 1956).
The validity of Brierley and Hemmings' conclusions is discussed by
Williams (1964).
Needham (1950) has calculated that during the early part of development the yolk sac absorbs over three times its own weight of yolk each
day. After the 9th day, however, it absorbs a quantity equivalent to its
own weight. In the early stages much of the absorbed material is utilized
for building the yolk sac itself.
E. The Yolk Sac as a Transitory Liver'
The concept that the yolk sac of birds acts as a glycogen store before
the liver has developed was originally put forward by Claude Bernard
(1888; see Needham, 1931, for discussion). I t was based on the fact that
during the early stages of development a great deal of glycogen was
present in the yolk sac and practically none was found in the liver.
Subsequently, at about 13 days of incubation the situation changed, the
liver suddenly acquiring more glycogen and the yolk sac losing most of
its store. These initial observations have been confirmed both histochemically and by chemical analysis (see Needham, 1931). In addition,
Zwilling ( 1952) has shown that the yolk sac resembles liver in being able to
increase its glycogen content if supplied with extra supplies of insulin.
As well as by storing glycogen, the yolk sac may act as a transitory
liver in other ways. For instance, the enzymes cysteine dehydrase and
lactic and malic dehydrogenases are present in the yolk sac in relatively
large amounts in the early stages, and in the liver in later development
(Solomon, 1958,1959,1963 ; see also Section VI, B). Similarly, glutamotransferase is very active in the yolk sac during early development,
though after 12-13 days its activity becomes greater in the liver
(RudnickefaZ., 1954).
259
Long arms of cytoplasm were occasionally found extended around large
yolk masses as if the cells have been fixed during the process of engulfing
yolk (Bellairs, 1963).
Trypan blue, if injected beneath the yolk sac, also enters the cells but
probably by diffusion rather than phagocytosis (Wislocki, 1921; Latta
and Busby, 1929).
Soluble
32
P when injected into the yolk sac is subsequently found in
the blood and body of the embryo (Taylor and Saenz, 1949). I t seems
possible that this too is due to diffusion rather than ingestion proper,
since the
32
P is not incorporated into the yolk phospholipids (Webster
and Ward, 1953). Similarly, although it has been found that viruses or
specific antibodies are taken up by the embryo after injection into the
yolk (Brandly et al., 1946 ; Brierley and Hemmings, 1956) the mechanism
is not properly understood, though it seems clear that the vitelline circulation is involved (Buxton, 1952; Brierley and Hemmings, 1956).
The validity of Brierley and Hemmings' conclusions is discussed by
Williams (1964).
Needham (1950) has calculated that during the early part of development the yolk sac absorbs over three times its own weight of yolk each
day. After the 9th day, however, it absorbs a quantity equivalent to its
own weight. In the early stages much of the absorbed material is utilized
for building the yolk sac itself.
E. The Yolk Sac as a Transitory Liver'
The concept that the yolk sac of birds acts as a glycogen store before
the liver has developed was originally put forward by Claude Bernard
(1888; see Needham, 1931, for discussion). I t was based on the fact that
during the early stages of development a great deal of glycogen was
present in the yolk sac and practically none was found in the liver.
Subsequently, at about 13 days of incubation the situation changed, the
liver suddenly acquiring more glycogen and the yolk sac losing most of
its store. These initial observations have been confirmed both histochemically and by chemical analysis (see Needham, 1931). In addition,
Zwilling ( 1952) has shown that the yolk sac resembles liver in being able to
increase its glycogen content if supplied with extra supplies of insulin.
As well as by storing glycogen, the yolk sac may act as a transitory
liver in other ways. For instance, the enzymes cysteine dehydrase and
lactic and malic dehydrogenases are present in the yolk sac in relatively
large amounts in the early stages, and in the liver in later development
(Solomon, 1958,1959,1963 ; see also Section VI, B). Similarly, glutamotransferase is very active in the yolk sac during early development,
though after 12-13 days its activity becomes greater in the liver
(RudnickefaZ., 1954).