Fig.18. Gastrulation in Birds

A – formation of the primitive streak and the primitive node: 1) the primitive node; 2) the primitive pit; 3) the primitive streak; 4) the primitive groove

B – formation of mesoderm (immigration).

More epiblast cells ingress and form the head mesoderm and the anterior part of the notochord. The notochord is formed from some of the mesodermal cells in a middle layer between the ectoderm and the endoderm along the primitive streak. Following this, the primitive streak regress, moving Hensen’s node posteriorly. As it moves, the more posterior regions of the notochord develop. At its most posterior position, Hensen’s node forms the anal region. Notochord establishes the future axis of the vertebral column.

In the process of gastrulation, mesenchyme, embryonic connective tissue is forming. Mesenchymal cells are mainly mesodermal in origin but may be derived from the other germ layers. So, the cells migrate from their site of origin, surrounding and penetrating developing organs. Mesenchyme consists of loosely arranged stellate cells suspended in a gelatinous matrix. Their processes make contact with those of adjoining cells.

  At the end we have three layers, the endoderm which has displaced the hypoblast, the ectoderm, which is what is left of the epiblast, and lots of mesoderm in between. Together the three germ layers and the mesenchyme give rise to all tissues, all organs and organ systems of the developing organism.

Thus, after gastrulation there follows a phase of histo- and organogenesis in which the primordial organs  make their appearance. This phase gradually merges into the periods of growth and differentiation - both histological and functional.

At the beginning the formation of the axial primordial organs takes place. The complex of the axial organs includes: notochord, neural tube, mesodermal somites (paraxial mesoderm).

Neurulation (Fig.19) is the development of the neural tube form the neuroectoderm.Nerve tissue develops from embryonic ectoderm that is induced to differentiate in this direction by the underlying notochord. At first, a neural plate forms; then the edges of the plate thicken, forming the neural groove. The edges of the groove grow toward each other and ultimately fuse, forming the neural tube. This structure gives rise to the entire central nerve system. Some cells lateral to the neural groove, making up the neural crest, undergo extensive migrations and give rise to most of the peripheral nervous system, as well as a number of other structures (melanocytes, medulla of the adrenal gland, cells of the diffuse endocrine system. The rest of intra-embryonic ectoderm gives rise to the skin epidermis and its derivatives.

Fig.19

The circumferential continuity of the endoderm is restored with the formation of a tube the definitive gut.

Alongside the neural tube and the notochord, mesoderm forms metamerically arranged somites (Fig.20). More laterally is a strip of intermediate mesoderm (syn.: nephrogonotom e, or segment stalk). Pairs of somites and nephrotomes are metamerically segmented portions of the mesoderm. The splanchnotome (or lateral plate mesoderm) is a non-segmented portion of the mesoderm, placed the most laterally.

                      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;