Extra-embryonic (provisory) organs
Extra-embryonic portions of the germ layers form extra-embryonic (provisory) organs.
Extra-embryonic organs provide environment for the embryo development and vital activity. They function during the period of embryogenesis and disappear with the end of the embryonic development, when foetus attains ability for independent existence.
In birds extra-embryonic organs are:
amnion;
serous membrane;
yolk sac;
allantois.
At the beginning the embryo throws itself flat on the yolk surface. With further growth the embryo raises itself over the yolk mass as result of the formation of the body folds. All germ layers (ectoderm, entoderm, visceral and parietal layers of splanchnotome) take participation in the body fold' formation: they are tucked under the embryo (Fig.21). So, the embryo is lifted off the yolk by the development of infoldings unite along the sides of the sides of the embryo. Thus, connection to the yolk is progressively is restricted.
Amniotic folds, forming by extra-embryonic portions of ectoderm and parietal layer of mesoderm (somatopleure), raise first above the cranial edge, and then - above the whole trunk of the embryo. The folds meet dorsal to the embryo (Fig.22). The ectoderm of one fold grows together with the ectoderm of another fold. The parietal layers of both folds fuse also. Thus, the extra-embryonic somatopleure subdivides into two membranes: the amnion, surrounding the amniotic cavity, and the serous membrane (chorion). The wall of the amniotic cavity is formed by the ectoderm, lining the cavity from the inside, and the parietal layer of mesoderm, lying on the outside. Ectodermal amniotic epithelium secrets amniotic fluid, so the amniotic cavity becomes filled with some fluid. The amnion provides a local aquatic environment for the embryo, eliminates gravitational stresses, the formation of adhesions and the danger of desiccation.
Fig.22. Extra-embryonic organs in Birds
amniotic cavity; 2) allantois; 3) yolk sac; 4) body fold; 5) extraembryonic ectoderm; 6) parietal layer of splanchnotome; 7) amnion wall; 8) amniotic folds joint; 9) serous membrane; 10) embryonic ectoderm; 11) notochord; 12) nerve tube; 13) dermatome; 14) myotome; 15) sclerotome; 16) nephrogonotome;
17) Aorta; 18) forming gut tube; 19) coelom; 20) blood vessels in the yolk sac wall.
The serous membrane is formed by extra-embryonic portion of the parietal layer of mesoderm (inside) and the ectoderm (outside). The serous membrane grows around the whole embryo and lines the shell. The function of the serous membrane is a respiratory exchange.
The extra-embryonic splanchopleure (entoderm + visceral layer of mesoderm) covers the yolk mass as the wall of yolk sac. During the body fold formation, the yolk sac becomes separated from the primitive gut. Yolk is transformed into soluble form under influence of enzymes, produced by endodermal cells. The dense net of blood vessels, developing in the wall of the yolk sac, transport nutrients derived from the soluble yolk, to the embryo. So, the function of the yolk sac is assimilation of the yolk.
The allantois arises as a diverticulum of the ventral wall of the gut' caudal part, and its wall is composed of the entoderm (inside) and visceral layer of mesoderm (outside) (Fig.23). The allantois expands, almost filling the extra-embryonic coelom, and its outer wall lines the serous membrane. Blood vessels in the wall of the expending allantois vascularize the overlying serous membrane. Respiratory exchanges occur between the vascularized membranes and the external air via the porous shell. The cavity of allantois serves as a receptacle for foetal urine.
