SYMMETRIZATION OF THE EGG OF VERTEBRATES 41
the shell membrane to the shell itself at either end by the albuminous
ligaments. Elsewhere the sac is separated from the shell by a fluid layer,
the outer liquid albumen layer, and from the yolk by the inner liquid
albumen layer. The chalazae are two coils of fibres, at the blunt and
sharp ends, each attached internally to the chalaziferous membrane and
externally to the albuminous sac. Normally the two chalazae are coiled
in opposite directions: when that of the blunt end coils to the left that
at the sharp end coils to the right and vice versa. The chalazae, roughly
stretching along the egg axis, constitute an axis of rotation around
which the yolk can rotate, maintaining the germ upwards, within the
inner layer of liquid albumen. The volume of liquid which makes up the
inner layer of liquid albumen is extremely variable and can, according
to the Romanoff's (1949) range proportionally from 1 to 40. Obviously
the quantity of this inner layer of liquid albumen controls yolk mobility,
which will be high, allowing the yolk to rotate quickly to its equilibrium
position, if the inner liquid albumen is voluminous, low if it is not. This
accounts for the individual differences among eggs.
B. Formation of the Egg
The yolk and its vitelline membrane are formed in the ovary, the
membranes that cover it are formed in the oviduct. Female birds, in
general, have only one functional oviduct, the left. This oviduct has
several segments: the infundibulum, the glandular part or magnum, the
isthmus, the uterus, and the vagina. Except for the vagina, all parts
have the same structure: they are lined by a simple epithelium with
two types of cells, mucous and ciliated. The mucosa possess tubular
glands which vary from one region to another. The genital tract is
differentiated and becomes functional under the influence of ovarian
endocrine secretions. When the ovary is active the different segments
store what they will secrete when the egg passes. Each region, as its
structure seems to suggest, builds up part of the egg membranes.
However the uterus, in which the egg remains longest, performs the
most delicate task. After follicular rupture, the egg is caught by the
infundibulum and after 15 minutes, on the average, it is driven into the
magnum through which it travels in about three hours, becoming invested by the albumen sac. According to Bartelmez (1918) and von
Baer (1828), the ovoid egg always enters the magnum in the same way,
with its major axis parallel to the length of the oviduct. Thus the germ
is always situated towards the periphery of the oviduct, and during its
passage, as the egg slowly rotates, the germ always remains peripheral.
When the egg reaches the isthmus it remains there for about an hour.
There it is enveloped by the shell membranes, thus definitely fixing its
shape. The egg then passes into the uterus where it remains for twenty
the shell membrane to the shell itself at either end by the albuminous
ligaments. Elsewhere the sac is separated from the shell by a fluid layer,
the outer liquid albumen layer, and from the yolk by the inner liquid
albumen layer. The chalazae are two coils of fibres, at the blunt and
sharp ends, each attached internally to the chalaziferous membrane and
externally to the albuminous sac. Normally the two chalazae are coiled
in opposite directions: when that of the blunt end coils to the left that
at the sharp end coils to the right and vice versa. The chalazae, roughly
stretching along the egg axis, constitute an axis of rotation around
which the yolk can rotate, maintaining the germ upwards, within the
inner layer of liquid albumen. The volume of liquid which makes up the
inner layer of liquid albumen is extremely variable and can, according
to the Romanoff's (1949) range proportionally from 1 to 40. Obviously
the quantity of this inner layer of liquid albumen controls yolk mobility,
which will be high, allowing the yolk to rotate quickly to its equilibrium
position, if the inner liquid albumen is voluminous, low if it is not. This
accounts for the individual differences among eggs.
B. Formation of the Egg
The yolk and its vitelline membrane are formed in the ovary, the
membranes that cover it are formed in the oviduct. Female birds, in
general, have only one functional oviduct, the left. This oviduct has
several segments: the infundibulum, the glandular part or magnum, the
isthmus, the uterus, and the vagina. Except for the vagina, all parts
have the same structure: they are lined by a simple epithelium with
two types of cells, mucous and ciliated. The mucosa possess tubular
glands which vary from one region to another. The genital tract is
differentiated and becomes functional under the influence of ovarian
endocrine secretions. When the ovary is active the different segments
store what they will secrete when the egg passes. Each region, as its
structure seems to suggest, builds up part of the egg membranes.
However the uterus, in which the egg remains longest, performs the
most delicate task. After follicular rupture, the egg is caught by the
infundibulum and after 15 minutes, on the average, it is driven into the
magnum through which it travels in about three hours, becoming invested by the albumen sac. According to Bartelmez (1918) and von
Baer (1828), the ovoid egg always enters the magnum in the same way,
with its major axis parallel to the length of the oviduct. Thus the germ
is always situated towards the periphery of the oviduct, and during its
passage, as the egg slowly rotates, the germ always remains peripheral.
When the egg reaches the isthmus it remains there for about an hour.
There it is enveloped by the shell membranes, thus definitely fixing its
shape. The egg then passes into the uterus where it remains for twenty