260
RUTH BELLAIRS
Clark and Fischer (1957) have also suggested that the yolk sac stores
nitrogenous excretory products during the first days of incubation.
According to them, the yolk sac converts ammonia to urea which is
subsequently transferred in solution to the allantois.
V I . Chemical Changes in the Yolk during Incubation
A. Uptake of Water
One of the most obvious changes during incubation is in the consistency of the yolk. This is the result not only of the activity of enzymes
(see below) but also of the uptake of water from the egg white. Needham (1931) summarized the then existing evidence to show that during
the first 10 days at least, water passes from the white to the yolk; it is
possible that after this period the water content of the yolk decreases.
He also concluded that the flow of water starts whilst the yolk is still in
the oviduct, as soon as it has become surrounded by the egg-white.
Bartelmez and Riddle (1924) showed that water uptake was greatest
during this period, and they suggested that the water was the source of
the sub-blastodermic fluid. The passage of fluid through the vitelline
membrane has also been demonstrated experimentally by New (1956).
The flow of water is partially the result of osmosis, but it appears that
the vitelline membrane also influences it. Needham (1931) found that if
the yolk was separated from the white by a parchment membrane
instead of the vitelline membrane, the water entered the yolk more
slowly. A further factor may be that in the normal egg the egg-white
proteins hold back a certain amount of water.
B. Enzyme Activity in the Yolk and Yolk Sac
A considerable battery of enzymes is already present in the laid egg,
some in the yolk and some in the blastoderm. The activity of these
enzymes however appears to be very low. Linneweaver et al. (1948) have
reported the presence of tributyrinase, peptidase, catalase, amylase and
phosphatase in the yolk. Ito (1957) found that two proteinases were
active, one at p H 2-8, and the other at p H 5-8 ; these probably correspond
with the proteinases described by Giudicci and Rocca (1959) which act
at these p H levels in incubated eggs. Emmanuelsson (1951) reported
that at least three proteinases were present in the unincubated yolk, and
concluded that at least one of them was active because free amino acids
were also present. This seems a surprising conclusion since some free
amino acids are present in all tissues. I t is possible, however, that some
proteinase activity occurs during the time when the egg is travelling
down the oviduct inside the warm body of the hen.
RUTH BELLAIRS
Clark and Fischer (1957) have also suggested that the yolk sac stores
nitrogenous excretory products during the first days of incubation.
According to them, the yolk sac converts ammonia to urea which is
subsequently transferred in solution to the allantois.
V I . Chemical Changes in the Yolk during Incubation
A. Uptake of Water
One of the most obvious changes during incubation is in the consistency of the yolk. This is the result not only of the activity of enzymes
(see below) but also of the uptake of water from the egg white. Needham (1931) summarized the then existing evidence to show that during
the first 10 days at least, water passes from the white to the yolk; it is
possible that after this period the water content of the yolk decreases.
He also concluded that the flow of water starts whilst the yolk is still in
the oviduct, as soon as it has become surrounded by the egg-white.
Bartelmez and Riddle (1924) showed that water uptake was greatest
during this period, and they suggested that the water was the source of
the sub-blastodermic fluid. The passage of fluid through the vitelline
membrane has also been demonstrated experimentally by New (1956).
The flow of water is partially the result of osmosis, but it appears that
the vitelline membrane also influences it. Needham (1931) found that if
the yolk was separated from the white by a parchment membrane
instead of the vitelline membrane, the water entered the yolk more
slowly. A further factor may be that in the normal egg the egg-white
proteins hold back a certain amount of water.
B. Enzyme Activity in the Yolk and Yolk Sac
A considerable battery of enzymes is already present in the laid egg,
some in the yolk and some in the blastoderm. The activity of these
enzymes however appears to be very low. Linneweaver et al. (1948) have
reported the presence of tributyrinase, peptidase, catalase, amylase and
phosphatase in the yolk. Ito (1957) found that two proteinases were
active, one at p H 2-8, and the other at p H 5-8 ; these probably correspond
with the proteinases described by Giudicci and Rocca (1959) which act
at these p H levels in incubated eggs. Emmanuelsson (1951) reported
that at least three proteinases were present in the unincubated yolk, and
concluded that at least one of them was active because free amino acids
were also present. This seems a surprising conclusion since some free
amino acids are present in all tissues. I t is possible, however, that some
proteinase activity occurs during the time when the egg is travelling
down the oviduct inside the warm body of the hen.