The olfactory placode sinks below the surface to form the floor of a depression called the nasal (olfactory) pit. Proliferating mesenchymal cells around the pit raise the surrounding ectoderm into medial and lateral nasal elevations. The olfactory placode becomes the olfactory epithelium under which mesenchymal cells collect to form the nasal capsule. The frontonasal area lies between as well as dorsal to the medial nasal elevations.
The maxillonasal (nasolacrimal) groove appears in the ectoderm between the lateral nasal elevation and the maxillary process. It extends from the medial side of the optic cup to the nasal pit.
The maxillary process is separated from the medial nasal elevation by an epithelial plate called the nasal fin.
The distal end of the hypophyseal pouch remains in close association with the neurohypophyseal bud of the diencephalon just caudal to the chiasmal area. The pouch elongates and flattens in the transverse plane. Its wall thickens and remains in continuity with the epithelial lining of the stomodeum. The lumen of the pouch communicates with the stomodeum through a narrow constriction that is closing.
FOREGUT
The pharyngeal pouches remain connected to the primitive pharynx. The first and second pouches dilate in close proximity to their respective branchial groove. The lining of the third and fourth pouches thickens, becomes removed from the grooves and divides into dorsal and ventral portions. The fifth pouch (ultimobranchial body) is incorporated into the fourth pouch.
The caudal segment of the thyroid diverticulum becomes a solid mass of cells that forms the thyroid gland. It is temporarily connected to the floor of the pharynx at the foramen cecum linguae by the thyroglossal duct.
Swellings appear in the floor of the primitive pharynx where the tongue develops. There are three such swellings rostral to the foramen cecum linguae at the level of the first branchial arch, a midline tuberculum impar flanked on each side by a mandibular swelling. Three similar swellings also form caudal to the foramen cecum linguae at the level of the second, third and fourth branchial arches, a midline hypobranchial eminence flanked on each side by a hyoid swelling.
Mesenchymal cells begin to form a condensed layer peripheral to the pharyngeal endodermal lining.
At the level of the septum transversum the esophagus passes to the left of the midline to terminate in the stomach. The endoderm of the stomach expands into a crescent shape with greater and lesser curvatures surrounded by a dense cellular condensation. The body and pyloric portions of the stomach become evident as rotation begins. The greater curvature (dorsal border) moves to the left and the lesser curvature (ventral border) moves to the right. The dorsal mesogastrium also swings to the left since it is attached to the greater curvature. Mesenchymal cells collect in its caudal portion forming a bulge that represents the primordium of the spleen.
The most caudal segment of the foregut does not dilate like the stomach segment and represents the cranial part of the duodenum. The dorsal pancreatic bud that arises from this segment branches as it invades the caudal part of the dorsal mesogastrium forming the dorsal pancreas. As the bud grows toward the left, it passes ventral to the superior mesenteric vein and terminates close to the splenic primordium.
RESPIRATORY SYSTEM
A third midline swelling appears caudal to the hypobranchial eminence at the level of the fourth branchial arch and represents the epiglottis. The laryngeal orifice caudal to the epiglottis is flanked on each side by arytenoid swellings. A cellular condensation surrounds the primitive larynx and the tracheal endodermal tube.
The trachea grows caudally and bifurcates in the caudal cervical segments into right and left primary bronchi. The right bronchus subdivides into upper, middle and lower lobar buds (secondary bronchi); the left bronchus subdivides into upper and lower lobar buds.
MIDGUT
The distal part of the hepatic diverticulum transforms into a complex of branching cords of cells (trabeculae) separated by venous channels (sinusoids), which together make up the liver. The liver lies in the most cranial part of the abdominal cavity and remains separated from the heart by the septum transversum. The proximal part of the hepatic diverticulum becomes the bile duct that empties into the fore- and midgut junction. The gall bladder forms as a diverticulum of the bile duct on the caudal surface of the liver. The bile duct then continues to the liver as the hepatic duct. The ventral pancreas is represented as a branching duct that moves first to the right side of the duodenum then to a dorsal position. It grows into the dorsal mesoduodenum and lies to the right of the portal vein. The ventral pancreatic duct joins the bile duct near the duodenum.
The yolk stalk disappears completely, separating the midgut from the yolk sac.
The midgut increases in length at such a rapid rate that it extends as a loop into the proximal part of the umbilical cord. The superior mesenteric artery courses through the dorsal mesentery to supply all of the midgut derivatives. The midpoint of the loop can be used to divide it into cranial and caudal limbs. The loop rotates counterclockwise around the artery with the cranial limb moving to the right and the caudal limb to the left.
A swelling forms in the caudal limb of the midgut that represents the primitive cecum.
The midgut endodermal tube is surrounded by a dense layer of mesenchymal cells.
HINDGUT
The cranial part of the hindgut remains in the midline suspended from the dorsal body wall by a mesentery through which courses the inferior mesenteric artery. The caudal end of the hindgut becomes the rectum and the urogenital sinus, which are separated by the urorectal septum. All of the hindgut derivatives with the exception of the urogenital sinus will be supplied by the inferior mesenteric artery.
The cloacal membrane becomes a vertical plate of epithelium separating the lumen of the cloaca, urogenital sinus and rectum from the exterior.
The tailgut disappears.
The hindgut endodermal tube becomes surrounded by a dense layer of mesenchymal cells.
Source: Atlas of Human Embryos.