of the blastocyst in the uterine endometrium begins at the end of the first week and is completed by the end of the second week.

The cellular and molecular events relating to             are complex.

The                             degenerates(day 5). Its disappearance results from enlargement of the blastocyst and degeneration caused by enzymatic lysis.

The              are released from the           of sperms that surround and partially penetrate the zona pellucida.

The             adheres to the endometrial epithelium.(day 6)

The trophoblast differentiates into two layers, the                  and the                .(day 7)

The                   erodes endometrial tissues and the blastocyst begins to embed in the endometrium. (day 8)

                  appear in the syncytiotrophoblast. (day 9)

The             sinks beneath the                     and the defect is filled by a closing plug. (day 10).

                   form by fusion of adjacent lacunae (day 10, 11)

The                erodes                  , allowing maternal blood to seep in and out of                         , thereby establishing a                       circulation. (day 11, 12)

The defect in the                     is repaired. (day 12, 13)

                                develop. (day 13, 14)

Rapid proliferation and differentiation of the                       occurs as the blastocyst completes implantation in the uterine endometrium.

The endometrial changes resulting from the adaptation of these tissues in preparation for implantation are known as the                              .

Concurrently, the primary umbilical vesicle forms and                             develops.

The extraembryonic coelom(cavity) forms from spaces that develop in the                                   .

The coelom later becomes the                          .

Concurrently, the                                 forms and extraembryonic mesoderm develops.

The extraembryonic           (          ) forms from spaces that develop in the extraembryonic mesoderm.

The            later becomes the chorionic cavity.

The amniotic cavity appears between the                            and                     .

The embryoblast differentiates into a                  embryonic disc consisting of                 , related to the amniotic cavity, and                 , adjacent to the blastocystic cavity.

The                            develops as a localized thickening of the hypoblast, which indicates the future               region of the embryo and the future site of the            ;

the                             is also an important organizer of the            region.