This distinction of layers of the bilaminar disc defines the primitive dorso ventral axis and polarity in embryogenesis.

The one-celled zygote, a eukaryotic cell formed by a fertilization event between two gametes at the start of embryonic development, undergoes cleavage by mitosis as it travels through the fallopian tube to the uterus.

[4] During implantation the blastocyst, which contains the inner cell mass, undergoes cellular differentiation into the two layers of the bilaminar embryonic disc.

The epiblast is the outer layer of the bilaminar embryonic disc and consists of columnar cells.

[6] Although the amniotic sac is initially smaller than the blastocyst it becomes larger by week eight until the entire embryo is encompassed by the amnion.

The main theory states that formation of the membranes of the yolk sac begins with an increase in production of hypoblast cells, followed by different patterns of migration.

[4] While the primary yolk sac is forming, extraembryonic mesoderm migrate into the blastocyst cavity and fill it with loosely packed cells.

This new cavity is responsible for detaching the embryo and, its amnion and yolk sac, from the far wall of the blastocyst, which is now named the chorion.

By day 13, the connecting stalk, a dense portion of extraembryonic mesoderm, restrains the embryonic disc in the gestational sac.

After the fourth week of development, the growing embryonic disc becomes much larger than the yolk sac and eventually involutes before birth.

Uncommonly, the yolk sac may persist as the vitelline duct and cause a congenital out pouching of the digestive tract called Meckel's diverticulum.

[7] During gastrulation, cells of the epiblast migrate towards the primitive streak, enter it, and then move apart from it through a process called ingression.