THE EMBRYONIC DEVELOPMENT IN BIRDS

Birds produce few, large, yolk-rich eggs with protective membranes and shells. Thus, the ova in birds are referred to as polylecithal, pronounced telolecithal ova.

Cleavage. During incomplete (meroblastic, partial), diskoidal cleavage of the telolecithal egg the yolk does not divide. Cytoplasmic divisions are limited to the animal pole of the zygote (Fig.16)/ A series of cleavage divisions result in appearance of blastomeres. Blastomeres are arranged in the form of a disk, which lies over the yolk mass. This blastula is termed as a diskoblastula. The roof of the diskoblastula is formed by the blastodisk, and the bottom is the superficial layer of non-dividing yolk, which is termed as periblast.

Fig.16.

Gastrulation in birds proceeds by combination of delamination and immigration.

The first phase of gastrulation is delamination(Fig.17); it results in splitting off the internal layer – hypoblast (primitive endoderm), which will give rise to the extra-embryonic endoderm. At first the gut endoderm consists of round or irregular-shaped cells, which are arranged disorderly. Gradually the cells close up and form more or less regular single-layered plate. The rest of the germ disk forms epiblast (primitive ectoderm). Subsequently a cleft arises, which distinctly separates hypoblast from epiblast. The blastocoele is the thin cavity between the epiblast and hypoblast.

Further, cells of the central part of epiblast proliferate intensively, close up tightly and become more highly-prismatic in shape, forming the thickening of the central part of blastodisk. The epiblast contains the cell material of presumptive epidermal ectoderm, notochord, neural tube and mesoderm and endoderm of the embryo body. The thickened part of the blastodisk, consisting of two layers (the external one - epiblast, and the internal one - hypoblast), is termed as the germ disk. The germ disk is just the material, which forms the embryo body. The marginal parts of the blastodisk are extra-embryonic ecto- meso- and endoderm.