YOLK OF THE HEN'S EGG
241
According to the observations of Bellairs and Greenfield, however, a
different modification appears to take place. There is some indication
(already noted by Schjeide et al., 1963a-c) that the lining body may, if it
projects sufficiently from the follicle cell, become nipped off in a vesicle
by the oocyte. Vesicles of this type have been seen as much as 1 μ deep in
the oocyte. The layers c and d appear to lose their clear outlines and
adhere to membrane b. In these vesicles the former intercellular space
between a and b now becomes packed with small granules, whereas the
large granules disappear from the inner layer of d. It appears, therefore,
that packets of material have been transferred to the embryo in a
vacuole, a conclusion which agrees with that of both Press (1959) and
Schjeide et al. (1963a-c), and are now broken down and perhaps released
into the cytoplasm (Fig. 14), The eventual fate of these densely lined
vacuoles is not clear though it seems not unlikely that they play a role
in preparing the oocyte for yolk formation. If this supposition is correct,
however, it is remarkable that the lining bodies are found even in the
infant chick of 7 days.
6. Granules
In an oocyte of about 1 mm diameter many vesicles are present peripheral to the cortical mitochondrial layer. The vesicles, which range in
size from about | μ to about \ μ diameter contain granules that are
individually about 100 Â, which is the usual size for R N P granules
(Palade, 1955). It is possible therefore, that the vesicles contain ribonucleo-proteins though there is no specific information yet available on
this point. Vesicles of this type are probably formed by pinocytosis from
the cell membrane of the oocyte.
In the early stages (up to about 1 mm) the cell membrane of the oocyte
lies close beneath the cell membrane of the follicle cell. Pinocytic-like
indentations, however, are visible in the cell membrane of the oocyte.
Later the zona striata (zona radiata) develops. This is a region visible in
sections examined by light microscopy as a band of striations around
the periphery of each oocyte (see Fig. 10). When examined by electron
microscopy, the zona striata appears to be formed of interdigitating
processes of follicle and oocyte cell surfaces (Fig. 15).
B. Oogenesis during Yolk Formation
It is only within the last thirty years that it has been possible to
understand where the raw materials for the yolk are manufactured.
There now seems to be general agreement that they are synthesized in
the liver and pass from there in the blood plasma to the ovary. This
explanation is supported by several lines of evidence. Firstly, the rate
of deposition of the yolk is too great for it to have been manufactured
241
According to the observations of Bellairs and Greenfield, however, a
different modification appears to take place. There is some indication
(already noted by Schjeide et al., 1963a-c) that the lining body may, if it
projects sufficiently from the follicle cell, become nipped off in a vesicle
by the oocyte. Vesicles of this type have been seen as much as 1 μ deep in
the oocyte. The layers c and d appear to lose their clear outlines and
adhere to membrane b. In these vesicles the former intercellular space
between a and b now becomes packed with small granules, whereas the
large granules disappear from the inner layer of d. It appears, therefore,
that packets of material have been transferred to the embryo in a
vacuole, a conclusion which agrees with that of both Press (1959) and
Schjeide et al. (1963a-c), and are now broken down and perhaps released
into the cytoplasm (Fig. 14), The eventual fate of these densely lined
vacuoles is not clear though it seems not unlikely that they play a role
in preparing the oocyte for yolk formation. If this supposition is correct,
however, it is remarkable that the lining bodies are found even in the
infant chick of 7 days.
6. Granules
In an oocyte of about 1 mm diameter many vesicles are present peripheral to the cortical mitochondrial layer. The vesicles, which range in
size from about | μ to about \ μ diameter contain granules that are
individually about 100 Â, which is the usual size for R N P granules
(Palade, 1955). It is possible therefore, that the vesicles contain ribonucleo-proteins though there is no specific information yet available on
this point. Vesicles of this type are probably formed by pinocytosis from
the cell membrane of the oocyte.
In the early stages (up to about 1 mm) the cell membrane of the oocyte
lies close beneath the cell membrane of the follicle cell. Pinocytic-like
indentations, however, are visible in the cell membrane of the oocyte.
Later the zona striata (zona radiata) develops. This is a region visible in
sections examined by light microscopy as a band of striations around
the periphery of each oocyte (see Fig. 10). When examined by electron
microscopy, the zona striata appears to be formed of interdigitating
processes of follicle and oocyte cell surfaces (Fig. 15).
B. Oogenesis during Yolk Formation
It is only within the last thirty years that it has been possible to
understand where the raw materials for the yolk are manufactured.
There now seems to be general agreement that they are synthesized in
the liver and pass from there in the blood plasma to the ovary. This
explanation is supported by several lines of evidence. Firstly, the rate
of deposition of the yolk is too great for it to have been manufactured