Digestive System The fetus consists of three layers ; ectoderm, mesoderm and endoderm.

Two folds, subcephalic fold cranially and subcaudal fold caudally are formed.

Develoment of mouth:

=An invagination of ectoderm appears termed stomodeum.

=The stomodium becomes in contact with the endoderm of foregut forming the oral membrane.

=The oral membrane ruptures

Derivatives of the stomodeum:-

1-Oral cavity 2-Teeth 3- salivary glands

5-glandular part (anterior lobe) of pituitary gland. Parts of primitive gut :- 1-Pharyngeal gut or pharynx extends from oral membrane to caudal to respiratory diverticulum. 2-Foregut extends from caudal to respiratory diverticulum to caudal to the liver bud. 3-Midgut extends from caudal to the liver bud to the junction of the right two thirds and left third of transverse colon . 4-Hindgut extends from junction of right two thirds and last third of transverse colon to the anal membrane.

Structure of digestive system:- The endoderm gives: - lining epithelium - glands of the digestive tract - liver and pancreas. The splanchnic mesoderm gives: muscular, connective tissue and peritoneal component of the wall of the gut. Pharynx Pharyngeal appartus :- -The endoderm in the head is costricted to form five pharyngeal pouches. -Four constrictions appear on the ectoderm forming pharyngeal grooves. -The mesoderm between endoderm and ectoderm forms pharyngeal arches. -The pharyngeal arches ,pouches and grooves together are called pharyngeal apparatus.

Pharyngeal pouches - five pairs of pharyngeal pouches. - First pharyngeal pouch:- - gives tubotympanic recess which forms and eustachian tube and tympanic cavity . Second pharyngeal pouch:- It forms the palatine tonsil. Third pharyngeal pouch:- The dorsal part gives inferior parathyroid gland - The ventral part gives thymus. Fourth pharyngeal pouch:- The dorsal part gives superior parathyroid gland. - The ventral part differentiated into thymus(10%). Fifth pharyngeal pouch:- - It gives ultimobranchial body which gives C-cells of the thyroid gland. - These cells secret calcitonin hormone involved in the regulation of calcium level in the blood.

Pharyngeal grooves (clefts): - There are four pharyngeal grooves. 1. The 1st groove gives external auditory meatus. The epithelium lying the meatus participate in the formation of the ear drum. 2. The second groove elongated ventrally forming cervical sinus which later on disappear. Differentiation of pharyngeal arches First pharyngeal arch ( Mandibular) arch: 1-Skeleton:- Mandible , some bones middle ear. 2-Muscles:- Muscles of mastication, tensor tympani and tensor veli palatini. 3-Nerves:- Mandibular nerve. Second pharyngeal arch:- 1-Skeleton:- Proximal part of hyoid bone and stapes of middle ear. 2-Muscles:- Muscles of expression and stapideus , stylohyoid and posterior belly of digastric muscle. 3-Nerves:- Facial nerve. Third pharyngeal arch:- Skeleton:- Distal part of the hyoid bone. Muscles: Stylopharyngeus aboralis ( Dilator). Nerves:- Glossopharyngeal nerve. Fourth pharyngeal arch:- Skeleton:- Cranial two cartilages of the larynx ( epiglottis and thyroid ) Muscles:- Constrictors of the pharynx, cricothyroid and levator veli palatini. Nerves:- Cranial laryngeal nerve of the vagus.

Fifth pharyngeal arch:- Skeleton:- caudal two cartilages of the larynx (arytenoid and cricoid) Muscles:- Intrinsic muscles of the larynx except → cricothyroid M. Nerves:-caudal laryngeal nerve of vagus.

Anomalies of pharyngeal apparatus 1-Preauricular cyst and fistula:

It is reminant of 1st pharyngeal groove .

2-Lateral cervical cyst:

on the lateral aspect of the neck just below the angle of the jaw.

3-First arch syndrome:

congenital malformations of eyes, ears, mandible and palate.

4-Lateral cervical ( branchial) fistula:

This fistula may be external or internal.

5-Congenital aplasia of thymus and parathyroid gland:

Fetuses are born without thymus and parathyroid gland. Tongue

Two lateral lingual swellings and a median swelling (tuberculum impar) develops from floor of first pharyngeal arch.

Fusion of 3 swellings forming body (anterior 2/3) of tongue and supplied by lingual nerve of mandibular nerve.

The second median swelling (copula) develops from floor of second, third pharyngeal arches .

The copula forms root (posterior1/3) of tongue, therefore the root of tongue is supplied by glossopharyngeal nerve. Anomalies of tongue:- 1-Short tongue 2-Long tongue 3-Microglossa → Due to atrophy of lingual muscles. 4-Macroglossa → Due to hypertrophy of lingual muscles ,Lingual thyroid and Thyroglossal cyst

5-Tongue tie ( Ankyloglossia) the fernulum linguae extends on the ventral aspect of the apex, the tongue cannot be protruded. 6-Bivit tongue:- Due to ununion between the two lateral lingual swellings. 7-Trivit tongue:- Due to failure of fusion between two lateral lingual swellings and tuberculum impar. Thyroid gland - The thyroid bud appears in the floor of the pharynx between the tuberculum impar and copula. - This bud extends caudally to give the thyroglossal duct and thyroid gland. - The thyroid bud extends caudally till reach the permanent position of thyroid gland, opposite to the first tracheal rings. Thenafter, the thyroglossal duct disappears. Anomalies of thyroid gland:- 1-Lingual thyroid:- -In which the thyroid gland persists in the tongue. 2-Thyroglossal fistula:- -The thyroglossal duct remains patent and opens in the floor of the tongue. 3-Thyroglossal cyst:- -It is remnant of thyroglossal duct. -It lies in midline of ventral aspect of neck under skin. -If this cyst opens it forms thyroglossal fistula. Esophagus - A small diverticulum (respiratory diverticulum) (laryngotracheal groove) appears at the ventral wall of the foregut . - This diverticulum is separated from the dorsal part of foregut through esophageotracheal septum. - The ventral portion ; respiratory primordium - dorsal portion ; the esophagus. Anomalies :- 1-Esophageal atresia: failure of esophageal recanalization. A newborn animal appears healthy but small in size and the first one or two swallows are normal , suddenly fluid return through the nose and mouth and respiratory complain. 2-Esophageotracheal fistula:- The esophagus is connected by a narrow canal with trachea just above the bifurcation . 3-Esophageotracheal ligament:- the fistulous canal between trachea and esophagus is replaced by a ligament. 4-Esophageal stenosis:- Results from incomplete recanalization of the esophagus. 5-Short esophagus :- esophagus is short, the stomach remains in the thoracic cavity Respiratory system. develops from respiratory diverticulum The proximal part becomes the larynx . The middle part elongates forming the trachea. The free end enlarged to form lung bud. The lung bud is subdivided into right and left lung buds The left lung bud is subdivided into two parts represents the cranial and caudal lobes . The right lung bud is subdivided into three parts represents the cranial lobe, caudal lobe and accessory lobe . Stomach The stomach appears as the most dilated part of the gut. The development of the stomach include: 1-Development of shape 2-Development of position Development of the shape: - the stomach appears spindle in shape - The dorsal border grows faster than the ventral border , producing the greater curvature and the ventral border becomes the lesser curvature. Development of the position: - There are two rotations:- 1.Around longitudinal axis 900 on left side, therefore the greater curvature becomes on the left side while the lesser curvature on the right side. 2. The second rotation takes place around transverse axis of stomach due to enlargement of the liver. therefore the greater curvature becomes on the left side and ventrally while lesser curvature becomes on the right side and dorsally as well as cardia on the left and pylorus on the right. Anomalies of stomach:- pyloric stenosis:- - due to hypertrophy of the circular muscle in the pyloric region . - It leads to severe narrowing of the pyloric part and obstruction of the passage of the food. - The vomits is strong and has acidic odour.