The nasal pit on each side of the midline deepens to form the primitive nasal cavity. The cavity opens to the exterior at the external nostril and is separated from its counterpart on the opposite side by the nasal septum. The olfactory epithelium lies in the roof of the cavity. A small epithelial invagination called the vomeronasal organ forms in the medial wall of the cavity on each side of the septum.
The stomodeum is replaced by the primitive oral cavity. The primitive nasal and oral cavities are initially separated by a thick oronasal membrane. The membrane soon thins and ruptures thereby communicating the two cavities through an opening called the primitive choana. The primitive choana enlarges in the roof of the primitive oral cavity on each side of the nasal septum. Rostral to the primitive choana the nasal septum joins an area of the primitive oral cavity roof called the primitive palate. The primitive palate is continuous with the medial nasal and frontonasal elevations. Because it lies between the maxillary processes, it is also called the intermaxillary segment. Remnants of the nasal fin are sometimes apparent between the primitive palate and maxillary process. As the lateral wall of the primitive oral cavity enlarges, the inner side of the maxillary process protrudes into the cavity forming the lateral palatine process. The rostral part of the lateral palatine process is in contact with the lateral surface of the tongue. Its caudal part lies below the tongue in the linguogingival groove where it remains until shortly before palate closure.
The nose begins to appear around the external nostrils when the frontonasal elevation becomes prominent and blends with the nasal elevations. The maxillonasal groove disappears after its walls fuse to produce the nasolacrimal duct. The duct courses from the medial aspect of the eye and terminates in the primitive nasal cavity.
The maxillary process blends with the nasal elevations. The mandibular process joins in the midline with its counterpart on the other side causing the median mandibular groove to disappear. The dorsal adjacent portions of the mandibular and maxillary processes blend to produce the cheek. Their ventral portions contribute to the formation of the primitive rima oris that replaces the stomodeal opening.
The roof and lateral walls of the primitive oral cavity grow rapidly to keep pace with the expansion of the rostral part of the brain. As a result the point of attachment of the hypophyseal pouch to the roof is shifted caudally to a position dorsal to the foregut floor. The junction of the pouch with the roof narrows to produce the solid stem remnant. Dorsal to the remnant the pouch retains its lumen and forms the adenohypophysis. The adenohypophysis becomes U shaped flanking each side of the neurohypophyseal bud. The lower connecting part of the U becomes the distal portion, the upper part on each side is the tuberal (infundibular) portion.
The epithelial lining of the primitive oral cavity in the vicinity of the maxillary and mandibular processes thickens in a U shaped area to form the labiodental lamina. The lamina separates into an inner dental lamina and an outer labiogingival band.
The parotid and submandibular buds appear as epithelial sprouts of the primitive oral cavity lining. The parotid bud begins in the cheek; the submandibular bud develops in the floor of the linguogingival groove.
FOREGUT
The tubotympanic recess (pouch 1) elongates in a dorsolateral direction to form the auditory tube and the primitive tympanic cavity. The second pouch reduces to the shallow palatine fossa. The third and fourth pouches lose their connections with the primitive pharyngeal wall. Their derivatives migrate into the ventral aspect of the neck. The dorsal portion of the third pouch becomes the caudal parathyroid bud, its ventral portion becomes the greater thymic bud. The two portions remain connected for a short period. The dorsal portion of the fourth pouch becomes the rostral parathyroid bud. The fate of its ventral portion is unclear but it probably contributes to the formation of the thymus (lesser thymic bud). The ultimobranchial body (pouch 5) is probably incorporated into the thyroid gland.
The thyroid gland loses its connection with the floor of the primitive pharynx and develops right and left lobes that are joined across the midline by the isthmus.
The swellings in the floor of the primitive pharynx blend to produce a massive tongue, which bulges into the primitive oral as well as pharyngeal cavities. The rostral part of the tongue becomes round and narrows to a tip near the primitive choanae.
The pharyngeal constrictor muscles become evident in the condensation peripheral to the endodermal lining. Loose mesenchymal tissue between the primitive pharynx and the vertebral column forms the retropharyngeal space. The caudal part of the primitive pharynx that passes dorsal to the larynx is called the laryngeal pharynx.
Caudal to the larynx the pharynx narrows to form the esophagus. The muscularis layer is apparent in the condensation surrounding the endodermal tube. Peripheral to the muscularis layer the vagus nerves communicate to form the esophageal plexus, which regroups at the caudal end of the esophagus to form the anterior (ventral) and posterior (dorsal) vagal trunks.
Stomach rotation is almost complete. The pyloric antrum is the slightly dilated portion to the left of the pylorus. The four layers of the stomach, mucosa (endodermal tube), submucosa, muscularis and serosa, become evident.
The boundaries of the spleen become discrete in the dorsal mesogastrium as it moves to a position dorsolateral to the stomach. The splenic blood vessels course through the dorsal mesogastrium to the spleen, stomach and dorsal pancreas.
The dorsal pancreas is represented by a branching duct that empties into the cranial part of the duodenum.
RESPIRATORY SYSTEM
Laryngeal cartilages begin to appear in the condensation around the laryngeal orifice. The orifice narrows to a vertical slit called the primitive glottis.
Tracheal cartilages begin to form in the condensation around the endodermal tube. The tracheal bifurcation migrates to the level of the upper thoracic segments. The secondary bronchi divided into numerous tertiary branches called bronchopulmonary buds near the periphery of the lung sac.
The lung sac begins to assume its definitive shape with a base at its caudal end and an apex at its cranial end. An oblique fissure in the left lung sac divides it into upper and lower lobes. In addition to the oblique fissure the right lung sac has a horizontal fissure. These two fissures divide the right lung sac into upper, middle and lower lobes.
MIDGUT
The liver increases tremendously in size enlarging more on the right than on the left. As the liver expands, the gall bladder enlarges and moves away from the bile duct. The proximal part of the gall bladder does not enlarge but becomes instead the cystic duct that empties the bladder into the bile duct.
The ventral pancreas moves to the left by passing dorsal to the duodenum. It is separated from the dorsal pancreas by the portal and superior mesenteric veins.
The duodenum assumes the shape of an arc coursing first dorsally and to the right, then caudally and to the left. It becomes continuous with the cranial limb of the midgut on the right side of the dorsal mesentery and superior mesenteric vessels. The lumen of the duodenum is partly obliterated in some areas but will become recanalized.
The midgut loop elongates at a rapid rate causing a greater portion to reside in the umbilical cord. It continues to rotate in a counterclockwise manner with the caudal limb passing to the left, above the cranial limb.
The primitive cecum lies in the proximal part of the umbilical cord and becomes the cecum after a diverticulum representing the appendix develops.
The muscularis of the midgut begins to differentiate from the mesenchymal condensation around the endodermal tube. It forms in the cranial limb before the caudal limb.
HINDGUT
The hindgut endodermal tube is surrounded by a thick condensation from which the muscularis differentiates. Differentiation of the muscularis in the hindgut lags behind that in the foregut and midgut.
When the urorectal septum joins the cloacal membrane, it divides the membrane into a ventral urogenital part and a dorsal anal part. The rectum terminates at the anal part, the urogenital sinus terminates at the urogenital part. See Section IV for discussion of the development of the urogenital sinus.
The area on each side of the anal membrane swells forming anal folds. The ectodermal depression between the folds is called the proctodeum.
Source: Atlas of Human Embryos.