The lateral palatine processes move to a position above the tongue where they fuse in the midline with each other and with the lower edge of the nasal septum. The processes thereby become the secondary palate that completes the separation between the primitive nasal and oral cavities. These cavities are then called nasal and oral cavities. The caudalmost part of the secondary palate that gives rise to the soft palate and uvula is usually the last to form.
The primitive palate differentiates into the median part of the upper lip and jaw and the triangular shaped median palatine process. The lateral aspect of the median palatine process fuses on each side with the rostralmost segment of the lateral palatine process. The median palatine process becomes the premaxillary palate caudal to which is the incisive foramen. Here the median and two lateral palatine processes come together in a Y shaped manner.
The external nostril through which the nasal cavity communicates to the exterior is temporarily occluded by an epithelial plug. The nasal cavity is continuous with the nasopharynx through the internal nostril (choana). The medial wall of the nasal cavity contains the vomeronasal organ, which eventually disappears. The longitudinal elevations in the lateral wall represent the superior, middle and inferior concha (turbinates). The nasolacrimal duct terminates below the inferior concha.
The oral cavity communicates with the exterior through the rima oris. The rima oris is surrounded by upper and lower lips that come together laterally at the angle. At the caudal part of the tongue the oral cavity is continuous with the oropharynx.
The distal portion of the adenohypophysis expands by forming branching epithelial cords between which is very vascular mesenchymal tissue. Subsequent expansion causes the residual lumen to disappear. The wall of the adenohypophysis adjacent to the neurohypophysis remains thin and is called the intermediate portion. The stem remnant disappears.
The dental lamina invades the underlying mesenchyme and forms 10 bell shaped enamel organs in each jaw that represent the beginning of the deciduous (milk) teeth. The enamel organ subsequently becomes indented on its deep side by a mesenchymal condensation called the dental papilla. Each enamel organ and dental papilla together make up a tooth bud. The outer enamel layer of a tooth is produced by the enamel organ. The inner dentine layer and the cementum are formed by the dental papilla. The buds for the permanent teeth form in a similar manner during the third month on the lingual aspect of the deciduous teeth. The labiogingival band also invades the jaw mesenchyme and becomes a sulcus that separates the lip and cheek from the gingiva or gum area. The sulcus will enlarge to become the vestibule of the oral cavity.
The distal segment of the parotid bud begins to branch around the facial nerve as it grows toward the preauricular area. It represents the parotid gland. The proximal segment becomes the parotid duct that in the adult empties into the upper part of the vestibule at the upper second molar tooth. The submandibular bud develops in a similar manner in the floor of the oral cavity giving rise to the submandibular gland and duct. The sublingual gland sprouts from the proximal part of the bud near the oral epithelium.
FOREGUT
The caudal part of the lateral palatine processes will subsequently fuse to form the soft palate and uvula that separate the nasopharynx from the oropharynx. The auditory tube (pouch 1) opens into the lateral wall of the nasopharynx. The more caudal palatine fossa (pouch 2) is located in the lateral wall of the oropharynx. The fossa will be invaded at a later period by lymphocytes that produce the palatine tonsil.
The greater thymic bud (pouch 3) migrates into the cranial part of the thorax where it joins its counterpart from the opposite side to form the thymus. The thymus comes to rest between the sternum and the left brachiocephalic vein but may extend as far cranially as the thyroid gland. As the greater thymic bud migrates caudally, it pulls the caudal (inferior) parathyroid bud (pouch 3) to a lower position than the rostral (superior) parathyroid bud (pouch 4). Each parathyroid bud will become a parathyroid gland on the dorsal surface of the thyroid gland.
The laryngeal pharynx continues caudally as the esophagus at the level of the cricoid cartilage. The four layers (mucosa, submucosa, muscularis and fibrosa) of the esophagus become apparent. The esophagus lies dorsal to the trachea and to the right of the aorta. As it approaches the diaphragm, it courses to the left in front of the aorta.
Stomach rotation is complete with the greater curvature facing caudally and the lesser curvature facing cranially. The stomach can be divided into three portions, fundus body and pyloric end. The pylorus develops at the gastroduodenal junction.
The dorsal pancreas becomes the body of the pancreas, which passes to the left to end as the tail at the spleen. The body subsequently fuses with the ventral surface of the left kidney.
RESPIRATORY SYSTEM
The thyroid cartilage forms as a plate of cartilage joined in the midline with its counterpart on the opposite side. It lies in front of the laryngeal orifice. The arytenoid cartilage appears in the arytenoid swelling dorsal to the orifice. The laryngeal cartilages (thyroid, arytenoid, corniculate and cuneiform) are derived from the condensations representing the fourth and fifth arch cartilages. The vocal folds (cords) grow medially from the lateral wall of the larynx and are separated by the glottis. The infraglottic cavity lies caudal to the vocal folds surrounded by the cricoid cartilage. The cricoid cartilage is considered to represent a derivative of a sixth arch cartilage.
The trachea begins at the cricoid cartilage and its walls are composed of a series of well-defined, U shaped cartilages. The caudal movement of the tracheal bifurcation slows and later attains the definitive level of T-4–T-5 vertebrae.
The bronchopulmonary buds (tertiary branches) branch repeatedly within the lung sac giving rise to the lung containing many generations of progressively smaller intrapulmonary bronchi. Approximately 18 generations are produced by the time of birth, the very small ones being called bronchioles. Between the sixth month and term, the distal ends of the terminal bronchioli expand to form the alveoli where gaseous exchange with the blood will take place after birth.
MIDGUT
The massive liver lies beneath the diaphragm and occupies approximately half of the abdominal cavity. It is very vascular and is the major site where blood cells are produced (hematopoiesis). Before birth hematopoietic activity in the liver subsides and moves to the red bone marrow.
The ventral pancreas becomes the head of the pancreas that joins the body around the superior mesenteric vein. Later the duct system of the dorsal and ventral pancreas fuse. The main pancreatic duct is formed from the ventral pancreatic duct and the distal segment of the dorsal pancreatic duct. It empties into the duodenum with the bile duct at the major papilla. The proximal segment of the dorsal duct either disappears or persists as the narrower accessory pancreatic duct that empties into the duodenum at the minor papilla.
All of the jejunum and most of the ileum arise from the cranial limb of the midgut loop. The caudal limb becomes the terminal segment of the ileum, the cecum with its appendix, the ascending colon and the right two-thirds of the transverse colon. The midgut loop remains in the umbilical cord until the third month when it returns to the abdominal cavity. The cranial limb returns first with the jejunum settling on the left and the ileum on the right. The caudal limb then returns. The cecum, appendix, ascending colon and descending colon initially lie from right to left in a horizontal position below the liver. The cecum and appendix gradually descend from the upper right to the lower right, pulling the ascending colon with it. See Section VI for fixation of the midgut derivatives to the dorsal body wall.
HINDGUT
When the midgut loop returns to the abdominal cavity, the cranial part of the hindgut is pushed to the left. It gives rise to the left one-third of the transverse colon, the descending colon and the sigmoid. The colon is very narrow initially but later will dilate to form the large intestine.
The terminal segment of the rectum gives rise to the upper part of the anal canal, which is therefore lined with endoderm. The lower part of the anal canal is derived from the proctodeum and is lined with ectoderm. The two parts of the anal canal become continuous with the disappearance of the anal membrane. The anal canal opens to the exterior at the anus. In the adult the point of attachment of the anal membrane is represented by the pectinate line. The pectinate line is the viscero-somatic junction with mucosa above and specialized skin below. The innervation, blood supply and lymphatic drainage differ for the two adjacent areas.
Source: Atlas of Human Embryos.