Endoderm; 12) blood vessels in the yolk sac wall

The splanchnotome splits onto parietal and visceral layers, which bound coelom cavities. The parietal layer of splanchnotome is in contact with ectoderm (somatopleure), and the visceral layer is in contact with endoderm (splanchnopleure). The lateral plate mesoderm gives rise to serous lining of cavities: pericardium, pleura, peritoneum, as well as cardiac muscle (myoepicardial plate, derived from the visceral layer of splanchnotome).

Mesenchyme cells are pluripotent, i.e. they may differentiate into a wide variety of cells. The mesenchyme gives rise to all types of connective tissue cells, as well as blood cells and blood vessels, hematopoietic organs, smooth muscle cells.

Endoderm gives rise to intestine epithelium and digestive glands (the liver, the pancreas), epithelial lining of the lungs, the urinary bladder etc.

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

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;