YOLK OF THE H E N ' S EGG
261
Subsequently, enzymes are also produced by the yolk sac (Borger and
Peters, 1933; Emmanuelsson, 1955). I t is often assumed that these
yolk sac enzymes are secreted into the yolk which they then break up
into smaller particles ready to be absorbed into the cells (see discussion
in Romanoff, 1960; Emmanuelsson, 1955). Some at least may, however,
act in situ on the intracellular yolk of the yolk sac (see Section III). I t
is not known whether yolk proteins are taken up as amino acids or as
protein molecules (see Williams, 1964, for discussion).
Unfortunately, most of the chemical analyses that have been carried
out throw little light on the problem. As Mclndoe (1960) has pointed
out, much of the work is difficult to assess as investigators have not
always made it clear whether the yolk sac was included in their estimates
for the embryo, or for the yolk, or was discarded. Moreover, the albumen sac has sometimes been included in yolk sac estimation (Needham,
1931). For this reason, it has not been possible in this discussion to distinguish between breakdown of the yolk that occurs inside the yolk sac
cells and that which takes place outside them.
1. Proteinases
Needham (1950) stated that during the first half of development, the
activity of the proteolytic enzymes is greater in the yolk sac than in the
embryo (Needham, 1950) and more proteins are absorbed than lipids.
According to Needham, much of this protein is used for the building of
the yolk sac itself. He also concluded, from a consideration of the
literature, that proteolytic activity increases during early incubation
and reaches a maximum at about 10 days. This has been confirmed by
recent workers (e.g. Emmanuelsson, 1955).
Several investigators have found that the concentration of free
amino acids increases up to about 9 days or more (Williams et al., 1954;
Flickinger, 1957), though Emmanuelsson (1955) was able to detect only
small quantitative changes up to 14 days of incubation. Quagliariello
et al. (1955), however, reported that whilst some amino acids increased
in quantities serine and threonine decreased.
Rupe and Farmer (1955) have shown by microbiological assay that
43% of the yolk substance is utilized by the embryo by about the 16th
day. The major part of the phosphoproteins have even disappeared
from yolk by the 10th-12th days. Until relatively recently, it was
thought that, unlike the eggs of amphibians, those of chicks possessed
no phosphoproteinphosphatase. However, Foote and Kind (1953)
described a method by which it could be extracted from hen's egg, and
they found that this enzyme did not become active until about 3 days
of incubation, when it was found in both yolk sac and embryo. Hasegawa et al. (1961) reported that phosphoproteinphosphatase completely
261
Subsequently, enzymes are also produced by the yolk sac (Borger and
Peters, 1933; Emmanuelsson, 1955). I t is often assumed that these
yolk sac enzymes are secreted into the yolk which they then break up
into smaller particles ready to be absorbed into the cells (see discussion
in Romanoff, 1960; Emmanuelsson, 1955). Some at least may, however,
act in situ on the intracellular yolk of the yolk sac (see Section III). I t
is not known whether yolk proteins are taken up as amino acids or as
protein molecules (see Williams, 1964, for discussion).
Unfortunately, most of the chemical analyses that have been carried
out throw little light on the problem. As Mclndoe (1960) has pointed
out, much of the work is difficult to assess as investigators have not
always made it clear whether the yolk sac was included in their estimates
for the embryo, or for the yolk, or was discarded. Moreover, the albumen sac has sometimes been included in yolk sac estimation (Needham,
1931). For this reason, it has not been possible in this discussion to distinguish between breakdown of the yolk that occurs inside the yolk sac
cells and that which takes place outside them.
1. Proteinases
Needham (1950) stated that during the first half of development, the
activity of the proteolytic enzymes is greater in the yolk sac than in the
embryo (Needham, 1950) and more proteins are absorbed than lipids.
According to Needham, much of this protein is used for the building of
the yolk sac itself. He also concluded, from a consideration of the
literature, that proteolytic activity increases during early incubation
and reaches a maximum at about 10 days. This has been confirmed by
recent workers (e.g. Emmanuelsson, 1955).
Several investigators have found that the concentration of free
amino acids increases up to about 9 days or more (Williams et al., 1954;
Flickinger, 1957), though Emmanuelsson (1955) was able to detect only
small quantitative changes up to 14 days of incubation. Quagliariello
et al. (1955), however, reported that whilst some amino acids increased
in quantities serine and threonine decreased.
Rupe and Farmer (1955) have shown by microbiological assay that
43% of the yolk substance is utilized by the embryo by about the 16th
day. The major part of the phosphoproteins have even disappeared
from yolk by the 10th-12th days. Until relatively recently, it was
thought that, unlike the eggs of amphibians, those of chicks possessed
no phosphoproteinphosphatase. However, Foote and Kind (1953)
described a method by which it could be extracted from hen's egg, and
they found that this enzyme did not become active until about 3 days
of incubation, when it was found in both yolk sac and embryo. Hasegawa et al. (1961) reported that phosphoproteinphosphatase completely