Fig.20. Differentiation of the germ layers in Birds

Ectoderm; 2) nerve tube; 3) somite; 4) nephrogonotome (segment stalk); 5) parietal layer of splanchnotome; 6) coelomic cavity; 7) visceral layer of splanchnotome; 8) splanchnotome (lateral mesoderm); 9) notochord; 10) endoderm; 11) blood vessels in the yolk sac wall.

Further development involves subsequent differentiation of the mesoderm. The somite divides into dermatome, myotome and sclerotome (Fig.21). The dermatome, outer mass of cells, gives rise to the connective tissue layer of the skin (the dermis) and subcutaneous tissues. Cells proliferating from the dorsal part of the inner wall of the somite form the myotome and give rise to somatic muscle. The cells of sclerotome (the ventral part of the inner wall) migrate medially, envelop the notochord and contribute to the axial skeleton and meninges.

The segmented stalks will give rise to the kidneys and gonads.

Fig.21. . Differentiation of the germ layers in Birds

1)ectoderm; 2) nerve tube; 3) dermatome; 4) myotome; 5) sclerotome; 6) nephrogonotome;

7) notochord; 8) aorta; 9) coelom; 10) amniotic fold; 11) extraembryonic ectoderm;

12) parietal layer of splanchnotome; 13) body fold; 14 ) visceral layer of splanchnotome;

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.