ABSTRACT
At the beginning of the 4th week after the last menstrual period, the implanted blastocyst is composed, from outside to inside, of the trophoblastic ring, the extra-embryonic mesoderm and the amniotic cavity and the primary yolk sac, separated by the bilaminar embryonic disk (Fig. 8.1). The extra-embryonic mesoderm progressively increases, and 12 days after ovulation (around the 26th menstrual day) it contains isolated spaces that rapidly fuse to form the extra-embryonic coelom. As the latter forms, the primary yolk sac decreases in size and the secondary yolk sac arises from cells growing from the embryonic disk inside the primary yolk sac.
