Y O L K OF T H E H E N ' S E G G
231
fore, that Revel et al. (1958) produced myelin figures by hydrating
lecithins obtained from chick yolk. It was concluded, therefore, that
the lamellated capsules seen around yolk spheres were also produced in
this way. The question of yolk hydration is discussed below (see p. 256).
(c) The unit membrane-like structure. In a minority of cases a unit
membrane| consisting of two dark lines separated by a light line was
present around a yolk sphere.
Recently, I have examined the yolk spheres of late oocytes and have
found that each is surrounded by a unit membrane. It seems, therefore,
that although it is normal to have a unit membrane around each yolk
sphere in the oocyte, it is usual to have lost it by the time the egg is
laid. This may be because the cytoplasmic components of the oocyte
separate off from the bulk of the yolk at fertilization so that the cytologically living unit membrane receives no more nourishment and
therefore perishes. Within the cells of the area pellucida, however, it
survives (see Section III).
III. The Formation of Yolk
The yolk is laid down during oogenesis. In the ovary of an adult hen
the oocytes are all intrafollicular, and in a regularly laying bird they
vary in size from as little as 50 μ to as much as 3*5 cm just before
ovulation. In a moulting or broody hen only small oocytes are present.
Potentially each little oocyte may grow into a big one. The process is,
however, slow and irregular until the oocyte is about 6 mm (Riddle,
1911) or perhaps 9 mm (Stieve, 1918) in diameter, but after that it takes
place with great speed. During the ensuing 8-10 days the oocyte reaches
its full size, having increased its diameter by an average of about 2-5
mm each day. This impressive growth rate is largely due to the deposition of yolk. I t is not clear at exactly what stage yolk begins to be laid
down, though most authors agree that this is before the final growth
phase.
The materials from which the yolk is formed are brought to the oocyte
by the blood stream (see below). It appears that the smallest oocytes
are incapable of utilizing the proteins, though it is possible that the
follicle cells are not permeable to them in the early stages.
Marza and Marza (1935) analysed the oocyte histochemically and
showed that its chemical composition changed during development.
They regarded these changes as evidence of variations in permeability
of the follicle cells and drew attention to three chemical phases of yolk
f 'Unit membrane' is the term introduced by Robertson (1959) to describe the appearance of normal cell membranes in electron micrographs. A unit membrane is about 75 A
wide and consists of two dark layers separate by a light layer (see Fig. 16 for membranes
i and ii).
231
fore, that Revel et al. (1958) produced myelin figures by hydrating
lecithins obtained from chick yolk. It was concluded, therefore, that
the lamellated capsules seen around yolk spheres were also produced in
this way. The question of yolk hydration is discussed below (see p. 256).
(c) The unit membrane-like structure. In a minority of cases a unit
membrane| consisting of two dark lines separated by a light line was
present around a yolk sphere.
Recently, I have examined the yolk spheres of late oocytes and have
found that each is surrounded by a unit membrane. It seems, therefore,
that although it is normal to have a unit membrane around each yolk
sphere in the oocyte, it is usual to have lost it by the time the egg is
laid. This may be because the cytoplasmic components of the oocyte
separate off from the bulk of the yolk at fertilization so that the cytologically living unit membrane receives no more nourishment and
therefore perishes. Within the cells of the area pellucida, however, it
survives (see Section III).
III. The Formation of Yolk
The yolk is laid down during oogenesis. In the ovary of an adult hen
the oocytes are all intrafollicular, and in a regularly laying bird they
vary in size from as little as 50 μ to as much as 3*5 cm just before
ovulation. In a moulting or broody hen only small oocytes are present.
Potentially each little oocyte may grow into a big one. The process is,
however, slow and irregular until the oocyte is about 6 mm (Riddle,
1911) or perhaps 9 mm (Stieve, 1918) in diameter, but after that it takes
place with great speed. During the ensuing 8-10 days the oocyte reaches
its full size, having increased its diameter by an average of about 2-5
mm each day. This impressive growth rate is largely due to the deposition of yolk. I t is not clear at exactly what stage yolk begins to be laid
down, though most authors agree that this is before the final growth
phase.
The materials from which the yolk is formed are brought to the oocyte
by the blood stream (see below). It appears that the smallest oocytes
are incapable of utilizing the proteins, though it is possible that the
follicle cells are not permeable to them in the early stages.
Marza and Marza (1935) analysed the oocyte histochemically and
showed that its chemical composition changed during development.
They regarded these changes as evidence of variations in permeability
of the follicle cells and drew attention to three chemical phases of yolk
f 'Unit membrane' is the term introduced by Robertson (1959) to describe the appearance of normal cell membranes in electron micrographs. A unit membrane is about 75 A
wide and consists of two dark layers separate by a light layer (see Fig. 16 for membranes
i and ii).