250
RUTH BELLAIRS
tissue culture with no other nourishment than glucose (Spratt, 1948, but
see Section VII). The yolk drops in the area pellucida gradually decrease
in number (Konopacka, 1933; Knorre, 1931; Lavarack, 1957; Bellairs,
1958, 1959) until after 48 h incubation very few complex or type A
drops are present. It seems probable, therefore, that the area pellucida
now receives food from the area opaca. Significantly, the yolk sac
circulation develops about this time so that a channel of communication
is open. It has been suggested in the literature that whole yolk drops
may pass from the area opaca to the area pellucida in this way (Schechtman, 1956). However, it seems more likely that the yolk in the area opaca
is broken down before it is transferred, since structural changes begin to
take place in it before the circulation is established (Bellairs, 1963). I t
is probable, therefore, that the yolk is transferred as small particles in
the blood.
V. The Yolk Sac
The yolk sac is a living organ whose main function is to absorb yolk
and pass it on to the growing embryo. To carry out this task it must
produce enzymes to break down the yolk, and blood vessels to transport it. It increases its efficiency by increasing its surface area, which it
does not only by spreading over almost the entire yolk, but also by
developing folds on its inner side which project into the substance of the
yolk. Its other function is to replace the vitelline membrane as a bag
enclosing the yolk; this takes place between about the 3rd and 4th day.
The lumen of the yolk sac is continuous with that of the gut, but it
appears that yolk does not enter the body by this route (see also
Section VIII).
A. The Origin of the Yolk Sac
The yolk sac is formed from endoderm which is continuous with the
gut, and from a layer of splanchnic mesoderm that is continuous with
the coelomic mesoderm (see Fig. 24). It develops from the area opaca or
germ wall.
At about 18 h of incubation (about stage 3 | of Hamilton, 1952) the
area opaca is said to consist of three concentric regions, the margin of
overgrowth, the syncytial zone and the inner zone (see Fig. 20). The
ectoderm is continuous throughout the three regions as an epithelium of
well-defined cells. Endoderm is absent from the marginal region but is
supposed to be 'syncytial' (but see below) in the 'syncytial' zone; it
consists of loosely arranged cells in the inner zone. Extra-embryonic
mesoderm is probably not present at this stage. In the next 48 h, it appears in the area opaca and begins to extend distally. This mesoderm
has probably migrated from the area pellucida though some authorities
RUTH BELLAIRS
tissue culture with no other nourishment than glucose (Spratt, 1948, but
see Section VII). The yolk drops in the area pellucida gradually decrease
in number (Konopacka, 1933; Knorre, 1931; Lavarack, 1957; Bellairs,
1958, 1959) until after 48 h incubation very few complex or type A
drops are present. It seems probable, therefore, that the area pellucida
now receives food from the area opaca. Significantly, the yolk sac
circulation develops about this time so that a channel of communication
is open. It has been suggested in the literature that whole yolk drops
may pass from the area opaca to the area pellucida in this way (Schechtman, 1956). However, it seems more likely that the yolk in the area opaca
is broken down before it is transferred, since structural changes begin to
take place in it before the circulation is established (Bellairs, 1963). I t
is probable, therefore, that the yolk is transferred as small particles in
the blood.
V. The Yolk Sac
The yolk sac is a living organ whose main function is to absorb yolk
and pass it on to the growing embryo. To carry out this task it must
produce enzymes to break down the yolk, and blood vessels to transport it. It increases its efficiency by increasing its surface area, which it
does not only by spreading over almost the entire yolk, but also by
developing folds on its inner side which project into the substance of the
yolk. Its other function is to replace the vitelline membrane as a bag
enclosing the yolk; this takes place between about the 3rd and 4th day.
The lumen of the yolk sac is continuous with that of the gut, but it
appears that yolk does not enter the body by this route (see also
Section VIII).
A. The Origin of the Yolk Sac
The yolk sac is formed from endoderm which is continuous with the
gut, and from a layer of splanchnic mesoderm that is continuous with
the coelomic mesoderm (see Fig. 24). It develops from the area opaca or
germ wall.
At about 18 h of incubation (about stage 3 | of Hamilton, 1952) the
area opaca is said to consist of three concentric regions, the margin of
overgrowth, the syncytial zone and the inner zone (see Fig. 20). The
ectoderm is continuous throughout the three regions as an epithelium of
well-defined cells. Endoderm is absent from the marginal region but is
supposed to be 'syncytial' (but see below) in the 'syncytial' zone; it
consists of loosely arranged cells in the inner zone. Extra-embryonic
mesoderm is probably not present at this stage. In the next 48 h, it appears in the area opaca and begins to extend distally. This mesoderm
has probably migrated from the area pellucida though some authorities