258
RUTH BELLAIRS
the yolk sac (Fig. 24). The albumen sac develops by the allantois growing round between the amnion and chorion and penetrating the final
point at which they are in contact, i.e. the sero-amniotic connection.
This region lies over the posterior end of the embryo. The wings of the
allantois then spread around and engulf the remains of the albumen sac.
A detailed description of the process is given by Romanoff (1960).
The lower border of the albumen sac becomes closed by the 13th day.
The breakdown of the sero-amniotic connection results in the albumen
entering the amniotic cavity.
The yolk is apparently prevented from mixing with the albumen
because of the presence of the remnants of vitelline membrane (Fig.
24). Eventually the yolk sac umbilicus closes between the 16th and
19th days of incubation. The yolk sac now remains open only at its
point of attachment to the embryo, that is, in the midgut region.
Gradually this opening becomes restricted until eventually a stalk is
produced and this attaches the yolk sac to the embryo. In birds, as in
other amniotes, part of the midgut herniates into the extra-embryonic
coelom. This is the part attached to the yolk sac stalk, and it is not taken
back into the body until a few days before hatching.
During its expansion over the surface of the yolk, the yolk sac develops a series of folds or ridges that hang down into the yolk. The main
folds radiate out from the embryo and extend in an animal-vegetal
direction. The principal omphalo-mesenteric vessels run along these
ridges. As development proceeds the ridges branch and become more
complex, and in places they fuse together so that the cavity of the yolk
sac becomes divided into a series of chambers. These ridges are found
only in the vascularized region of the yolk sac (see Romanoff (1960) for
a review of the literature, and for a detailed account of the growth of the
vascular network).
D. The Ingestion of Yolk by the Yolk Sac and the Embryo
I t is generally assumed that the extracellular yolk enters the cells by
being phagocytosed, though this does not appear to have been proved
directly either by the use of labelled substances or by filming the
activities of the yolk sac endoderm. All three germ layers of the chick
are apparently capable of phagocytic activity (Schechtman, 1956),
however, and the yolk sac is no exception. If Indian ink or carmine
particles are injected into the egg they are taken up by the yolk sac
(Heine, 1936; Schaper, 1902; Menkes, 1957; Krichinskaya, 1955).
Young blastoderms growing in tissue culture will take up particles of
colloidal gold or carmine (Bellairs and New, 1962).
Some indirect evidence for the possibility that phagocytosis occurs
was found when the area vitellina was examined by electron microscopy.
RUTH BELLAIRS
the yolk sac (Fig. 24). The albumen sac develops by the allantois growing round between the amnion and chorion and penetrating the final
point at which they are in contact, i.e. the sero-amniotic connection.
This region lies over the posterior end of the embryo. The wings of the
allantois then spread around and engulf the remains of the albumen sac.
A detailed description of the process is given by Romanoff (1960).
The lower border of the albumen sac becomes closed by the 13th day.
The breakdown of the sero-amniotic connection results in the albumen
entering the amniotic cavity.
The yolk is apparently prevented from mixing with the albumen
because of the presence of the remnants of vitelline membrane (Fig.
24). Eventually the yolk sac umbilicus closes between the 16th and
19th days of incubation. The yolk sac now remains open only at its
point of attachment to the embryo, that is, in the midgut region.
Gradually this opening becomes restricted until eventually a stalk is
produced and this attaches the yolk sac to the embryo. In birds, as in
other amniotes, part of the midgut herniates into the extra-embryonic
coelom. This is the part attached to the yolk sac stalk, and it is not taken
back into the body until a few days before hatching.
During its expansion over the surface of the yolk, the yolk sac develops a series of folds or ridges that hang down into the yolk. The main
folds radiate out from the embryo and extend in an animal-vegetal
direction. The principal omphalo-mesenteric vessels run along these
ridges. As development proceeds the ridges branch and become more
complex, and in places they fuse together so that the cavity of the yolk
sac becomes divided into a series of chambers. These ridges are found
only in the vascularized region of the yolk sac (see Romanoff (1960) for
a review of the literature, and for a detailed account of the growth of the
vascular network).
D. The Ingestion of Yolk by the Yolk Sac and the Embryo
I t is generally assumed that the extracellular yolk enters the cells by
being phagocytosed, though this does not appear to have been proved
directly either by the use of labelled substances or by filming the
activities of the yolk sac endoderm. All three germ layers of the chick
are apparently capable of phagocytic activity (Schechtman, 1956),
however, and the yolk sac is no exception. If Indian ink or carmine
particles are injected into the egg they are taken up by the yolk sac
(Heine, 1936; Schaper, 1902; Menkes, 1957; Krichinskaya, 1955).
Young blastoderms growing in tissue culture will take up particles of
colloidal gold or carmine (Bellairs and New, 1962).
Some indirect evidence for the possibility that phagocytosis occurs
was found when the area vitellina was examined by electron microscopy.