220
RUTH BELLAIRS
is possible that the haphazard production of the splits may be responsible for the different interpretations about the number of layers.
The inner layer of the vitelline membrane stains poorly with osmiumtetroxide but densely with phosphotungstic acid. It is about 3 μ thick
and consists of a network of solid fibres (see Figs. 2 and 3). The fibres
themselves are 0-2-0-6 μ in diameter. Sometimes they appear to have a
substructure of fine fibrils, each about 300 Â thick and up to £ μ or more
in length. The significance of these fibrils is unknown and attempts to
produce them by varying the preparative procedure were unsuccessful.
Similar fibrils have been seen in the vitelline membrane of doves and
turkeys (Mrs. Cynthia Jensen, personal communication). The holes
between the fibres vary in size but often measure about 2 μ in diameter.
The material which lies between them is probably yolk granules.
The outer layer is very variable in thickness. In unstained, unsectioned pieces of membrane, thick fibres can be seen even with the naked
eye, running in all directions parallel to the surface, and in places
branching. The fibres are commonest in the thickest part of the membrane. The chalazae apparently consist of twisted strands of these fibres
wrapped around with albumen. Electron micrographs show that the
individual fibres are composed of bundles of fibrils, the finest of which
are about 150 Â in diameter (Fig. 4) and are interlaced with one another
to form parallel sheets. The outer layer of the vitelline membrane is
thus composed of layer upon layer of these sheets of fibrils, as can be
seen in transverse sections (Fig. 2). The fibrils are not striated.
The 'continuous membrane' is composed of granules each about 70 Â
in diameter.
The chemical nature of the vitelline membrane has probably attracted
more attention than its physical structure, and in most textbooks it is
said to be formed of an inner layer of collagen and an outer layer of
mucus. This is based on the staining experiments of McNally (1943);
similar conclusions have recently been drawn from analysis of fixed and
stained material by Doran and Mueller (1961). Other interpretations
have been put forward at various times, however. For instance, investigators who have mainly examined fixed and stained material by
light microscopy have said that it is composed of keratin alone (Liebermann, 1888), of an inner layer of keratin and an outer one of mucus
(Moran and Hale, 1936), or of two fibrous layers and a cellular one
(Lécaillon, 1910). Shalumovitch (1955), who used ultraviolet and fluorescence microscopy as well as histochemistry, reported finding elastic
fibres as well as ribonucleic acid (RNA) and deoxyribonucleic acid
(DNA).
Inspection of the electron micrographs shows clearly that no collagen
is present in the vitelline membrane (Figs. 2, 3 and 4). Collagen is
RUTH BELLAIRS
is possible that the haphazard production of the splits may be responsible for the different interpretations about the number of layers.
The inner layer of the vitelline membrane stains poorly with osmiumtetroxide but densely with phosphotungstic acid. It is about 3 μ thick
and consists of a network of solid fibres (see Figs. 2 and 3). The fibres
themselves are 0-2-0-6 μ in diameter. Sometimes they appear to have a
substructure of fine fibrils, each about 300 Â thick and up to £ μ or more
in length. The significance of these fibrils is unknown and attempts to
produce them by varying the preparative procedure were unsuccessful.
Similar fibrils have been seen in the vitelline membrane of doves and
turkeys (Mrs. Cynthia Jensen, personal communication). The holes
between the fibres vary in size but often measure about 2 μ in diameter.
The material which lies between them is probably yolk granules.
The outer layer is very variable in thickness. In unstained, unsectioned pieces of membrane, thick fibres can be seen even with the naked
eye, running in all directions parallel to the surface, and in places
branching. The fibres are commonest in the thickest part of the membrane. The chalazae apparently consist of twisted strands of these fibres
wrapped around with albumen. Electron micrographs show that the
individual fibres are composed of bundles of fibrils, the finest of which
are about 150 Â in diameter (Fig. 4) and are interlaced with one another
to form parallel sheets. The outer layer of the vitelline membrane is
thus composed of layer upon layer of these sheets of fibrils, as can be
seen in transverse sections (Fig. 2). The fibrils are not striated.
The 'continuous membrane' is composed of granules each about 70 Â
in diameter.
The chemical nature of the vitelline membrane has probably attracted
more attention than its physical structure, and in most textbooks it is
said to be formed of an inner layer of collagen and an outer layer of
mucus. This is based on the staining experiments of McNally (1943);
similar conclusions have recently been drawn from analysis of fixed and
stained material by Doran and Mueller (1961). Other interpretations
have been put forward at various times, however. For instance, investigators who have mainly examined fixed and stained material by
light microscopy have said that it is composed of keratin alone (Liebermann, 1888), of an inner layer of keratin and an outer one of mucus
(Moran and Hale, 1936), or of two fibrous layers and a cellular one
(Lécaillon, 1910). Shalumovitch (1955), who used ultraviolet and fluorescence microscopy as well as histochemistry, reported finding elastic
fibres as well as ribonucleic acid (RNA) and deoxyribonucleic acid
(DNA).
Inspection of the electron micrographs shows clearly that no collagen
is present in the vitelline membrane (Figs. 2, 3 and 4). Collagen is