VAN 211: Veterinary Histology and Embryology (2+2)

GASTRULATION IN CHICK

• Gastrulation is confined to the blastoderm, which contains the cells of all the three future germ layers. The process, as a whole, is accomplished in two stages.
• The gastrula is formed by a process involution of the blastoderm. The caudal part of blastoderm rolls and tucks itself under the blastodermal cells. It divides until a new inner layer, the endoderm and an outer layer, the ectoderm are formed. The region where the ectoderm and endoderm meet represent dorsal lip of the blastopore of amphioxus. The space between the endoderm and the yolk is the archenteron.
• At the end of gastrulation a median band appear posteriorly on the surface of the blastoderm. It is a linear thickening of ectoderm and acquires an anterior knob called primitive knot. The thickening is called as the primitive streak which encloses a primitive groove and close to the primitive knot the primitive pit.
• The major function of the primitive streak is the formation of the third germ layer, the mesoderm. The adjacent cells on the surface of the blastoderm move medially enter the primitive streak and leave it as mesodermal cells.
• Soon the germinal disc is comprised of an outer ectoderm and an inner endoderm and the mesoderm, which is between the ectoderm and endoderm. Due to the migration of cells from the primitive streak on both the sides, a middle pole like invagination occurs on the median line of the streak, giving rise to the primitive groove folded by the primitive folds. From the primitive knot, a column of cells arise from the head process which becomes the notochord.