Medicine and Health related Information

At one circular area near the margin of the disc the cubical cells of the endoderm become columnar. This area is called “PROCHARDAL PLATE.” The appearance of the prochardal plate determines the central axis of the embryo and also enables us to distinguish its head and tail ends.

Soon after the formation of the prochardal plate some of the ectodermal cells lying along the central axis near the tail end of the disc begin to proliferate and form an elevation that bulges into the amniotic cavity. This elevation is called primitive streak. The primitive streak is at first a rounded or oval swelling, but with elongation of embryonic disc it becomes a linear structure lying in the central axis of the disc.

The intraenbryonic mesoderm spreads through out the disc except in the region of prochardal plate. The mesoderm extends on cranial to the prochordal plate, and here mesoderm from the two sides becomes continous across the midline. In the region of the prochordalplate the ectoderm and endoderm remain in contact. In later development the ectoderm and endoderm mostly persists as a living epithelium. On the other hand the bulk of the tissues of the body is formed predominantly from mesoderm. As there is no mesoderm in the prochardal plate, this region remains relatively thin and later forms ” BUCCOPHARNGEAL MEMBRANE”.

The prmitive streak gradually elongates along the central axis of the embryonic disc. The disc also elongates and becomes pear shaped.