At the beginning of the fourth week, folding in the median and horizontal planes converts the flat trilaminar embryonic disc into a C-shaped,                       .

The formation of the head, caudal eminence, and lateral folds is a continuous sequence of events that results in a constriction between the             and                       .

As the head folds            , part of the                 layer is incorporated into the developing embryonic head region as the            .

Folding of the head region also results in the oropharyngeal membrane and heart being carried            and the developing brain becoming the most cranial part of the embryo.

As the caudal eminence folds            , part of the endodermal germ layer is incorporated into the caudal end of the embryo as the            .

The terminal part of the hindgut expands to form the            .

Folding of the caudal region also results in the cloacal membrane, allantois, and connecting stalk being carried to the             surface of the embryo.

Folding of the embryo in the horizontal plane incorporates part of the endoderm into the embryo as the        .

The                remains attached to the midgut by a narrow                    (           ).

During folding of the embryo in the horizontal plane, the primordia of the               and               body walls are formed.

As the            expands, it envelops the connecting stalk, omphaloenteric duct, and allantois, thereby forming an epithelial covering for the               .

The three germ layers differentiate into various             and           , so that by the end of the embryonic period, the beginning of the main organ systems have been established.

The external appearance of the embryo is greatly affected by the formation of the          ,           , liver, somites, limbs, ears, nose, and eyes.

Because the beginning of most essential external and internal structures are formed during the        to       weeks, this is the most critical period of development. Developmental disturbances during this period may give rise to major                    

Reasonable estimates of the age of embryos can be determined from the day of onset of the           , the estimated time of                    , ultrasound measurements of the                       and embryo, and examination of external characteristics of the embryo.