What are differences between formation of embryonic disc in chick and mammal embryo?
Embryonic disc forms on top of yolk during cleavage on chicken embryo, while between amniotic sac and yolk sac inside blastocyst after implantation on human embryo.



Some further processes take place on and in the embryonic disc: specifically these here during the week 3.

Embryonic disc formation includes still during the 3rd week of development:


Formation of the primitive streak
Genesis of the germ layers
Genesis of the notochord


Formation of the primitive streak: The process of primary neurulation appears to be similar in chick and mammalian. The embryonic disc becomes oval and then pear-shaped, the wider end being directed forward. Near the narrow, posterior end, the primitive streak makes its appearance and extends along the middle of the disc for about one-half of its length from migrated ectoderm cells.



Genesis of the notochord: “The notochord extends beneath the neural tube from the base of the head into the tail.” - As the mesoderm develops between the ectoderm and endoderm it is separated into lateral halves by the neural tube and notochord.

Figure 2: Neurulation. NB the use of neural groove is correct. Notochord is below neural groove. - Genesis of germ layers here: from a flat thing to a developed one.



Figure 3: Genesis of notochord and primitive streak. Yolk-sac surrounds notochord and primitive streak.