The bilaminar embryonic disc is converted into a                         during              .

These changes begin with the appearance of the               , which appears at the beginning of the third week as a thickening of the epiblast at the               of the embryonic disc.

The                 results from migration of epiblastic cells to the median plate of the disc.

                    of epiblastic cells from the primitive streak give rise to              cells that migrate ventrally, laterally, and cranially between the             and            .

As soon as the primitive streak begins to produce                  , the epiblast is called                  .

Some cells of the epiblast displace the hypoblast and form                    .

                  produced by the primitive streak soon organize into a third germ layer, the                             , occupying the area between the former hypoblast and cells in the epiblast.

Cells of the            migrate to the edges of the embryonic disc, where they join the                  covering the                 and                           .

At the end of the third week, the embryo is a flat ovoid embryonic disc.

Mesoderm exists between the ectoderm and endoderm of the disc everywhere except at the                     , in the median plane occupied by the                  , and at the                       .

Early in the third week, mesenchymal cells from the primitive streak forms the                 between the embryonic ectoderm and endoderm.

The notochordal process extends from the primitive node to the                  .

Openings develop in the floor of the                 , and they soon coalesce, leaving a                .

This plate infold to form the            , the primordial axis of the embryo around which the              forms (eg. vertebral column).

The                appears as a thickening of embryonic ectoderm, induced by the developing                   .

A longitudinal                develops in the neural plate, which is flanked by              .

Fusion of the folds forms the             , the primordium of the            .

As the neural folds fuse to form the neural tube, neuroectodermal cells form a                 between

the surface ectoderm and neural tube.

The mesoderm on each side of the notochord condenses to form longitudinal columns of                         , which, by the end of the third week, give rise to            .

The              within the embryo arises as isolated spaces in the lateral mesoderm and cardiogenic mesoderm.

The                 subsequently coalesce to form a single, horseshoe-shaped cavity that eventually give rise to            .

               first appear in the wall of the umbilical vesicle, allantois, and chorion.

They develop within the embryo shortly thereafter.

                            develop from different hematopoietic precursors.

Blood vessels first appear in the wall of the                  ,                 , and              .

They develop within the embryo shortly thereafter.

Fetal erythrocytes develop from different hematopoietic precursors.

The                   is represented by paired endocardial heart tubes.

By the end of the third week, the heart tubes have fused to form a                that is joined to vessels in the embryo, umbilical vesicle, chorion, and connecting stalk to form a                                  .

The premordial heart is represented by                                          .

By the end of the third week, the heart tubes have fused to form a tubular heart that is joined to vessels in the                   ,                      ,                , and                           to form a primordial cardiovascular system.

Primary chorionic villi become                        as they acquire mesenchymal cores.

Before the end of the third week, capillaries develop in the secondary chorionic villi, transforming them into                       .

Cytotrophoblastic extensions from the stem villi join to form a                        that anchors

the chorionic sac to the endometrium.

Primary chorionic villi become secondary chorionic villi as they acquire                               .

Before the end of the third week,                   develop in the secondary chorionic villi, transforming them into                       .

Cytotrophoblastic extensions from the stem villi join to form a                        that anchors

the chorionic sac to the endometrium.