Fig.22. Extra-embryonic organs in Birds

1) 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;

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.

Fig.23. Formation of extraembryonic organs in Birds

EARLY STAGES OF HUMAN EMBRYOGENESIS

Egg cell. In eutherian mammals' ova yolk has been secondary reduced to the oligolecithal form because their prenatal development occurs in a uterus. Thus, the human ovum is classified as the secondary oligolecithal, isolecithal typeof ova.