An ectodermal depression called the stomodeum develops in the ventral part of the head region in the center of the developing face.
The mandibular process of the first branchial arch joins its counterpart on the other side in the midline to form the floor or caudal wall of the stomodeum. The two processes are separated on the surface by the median mandibular groove.
The maxillary process of the first branchial arch makes up the side or lateral wall of the stomodeum and is separated from the mandibular process by the primitive oral fissure.
The roof or rostral wall contains an evagination, the hypophyseal or Rathke’s pouch. The ectodermal lining of the pouch is adjacent to the neurohypophyseal area of the hypothalamus. An ectodermal thickening called the olfactory placode develops in the roof on each side, lateral to the telencephalic part of the prosencephalon.
FOREGUT
The oropharyngeal membrane ruptures and quickly disappears. As a result the endodermal lining of the foregut becomes continuous with the ectodermal lining of the stomodeum.
Five pharyngeal pouches appear on each side as blind, lateral extensions of foregut endoderm. This part of the foregut forms the primitive pharynx. Each of the first four pouches comes into contact with the ectoderm lining its corresponding branchial groove on the outside. The area of contact or closing membrane separates the pouch from the groove. The fifth pouch is actually a caudal extension of the fourth and is called the ultimobranchial body.
The point of attachment of the midline thyroid diverticulum moves slightly caudally to the level of the second aortic arch. The diverticulum becomes a small mass of cells adjacent to the aortic sac.
Respiratory system—The laryngotracheal groove deepens, converting the midportion of the foregut into a narrow tracheoesophageal tube that is compressed in the vertical plane. Behind the heart the tube separates into a small, dorsal primitive esophagus and a large, ventral primitive trachea. After a very short caudal course the primitive trachea separates into right and left lung buds.
The primitive esophagus is the caudal continuation of the foregut and terminates in a dilated segment of the endodermal tube, the primitive stomach.
Just caudal to the primitive stomach a small, dorsal midline diverticulum, the dorsal pancreatic bud, becomes evident.
MIDGUT
A ventral, midline outpouching, the hepatic diverticulum, appears at the fore- and midgut junction, or primitive duodenum. The proximal part of the diverticulum dilates into the hepatic antrum, which communicates with the lumen of the primitive duodenum. Hepatic trabeculae, or cords of cells, extend from the antrum on both sides into the mesoderm of the septum transversum. The trabeculae are separated from each other by venous channels called hepatic sinusoids. The trabeculae join with one another between sinusoids.
A small outpouching, the ventral pancreatic bud, extends caudally from the hepatic antrum.
Between the hepatic antrum and the yolk stalk the midgut elongates in the vertical plane. The midportion of the midgut communicates with the yolk sac ventrally through the narrow yolk stalk. Caudal to the yolk stalk the midgut endoderm takes the shape of a small, cylindrical tube with a barely visible lumen. It is surrounded by the splanchnic layer of lateral mesoderm and is suspended in the coelom by a mesentery through which blood vessels reach the gut.
Because there are no distinguishing characteristics at this time, the mid- and hindgut junction becomes an arbitrary point on the caudal segment of gut.
HINDGUT
The cranial part of the hindgut extends caudally, then ventrally to become continuous with the rectal portion of the cloaca. It also is suspended from the dorsal body wall by a mesentery.
The caudal part of the hindgut dilates into a large chamber called the cloaca. The cloacal endoderm is elongated in the dorsoventral plane and makes contact with ectoderm in the midline of the future external genitalia. The area of contact is called the cloacal membrane.
A midline mass of mesoderm called the urorectal septum begins to divide the cloaca into two parts: a) a ventral urogenital sinus and b) a dorsal rectum.
The hindgut ends blindly in the tail bud as the tailgut.
Source: Atlas of Human Embryos.