Note: Digestive System = mouth, pharynx, esophagus, stomach, small intestine (duodenum, jejunum,
& ileum), large intestine (cecum, colon & rectum) and anal canal; also, liver, pancreas, &
salivary glands. Mouth and salivary glands are presented under Head Development.

Early Development — foregut, midgut, and hindgut formation
• formation of the head process, tail fold, and ventral merger of lateral body folds during
early embryogenesis transforms flattened splanchnopleure into a blind foregut,
a blind hindgut, and a midgut connected to the yolk sac
• endoderm becomes the lining epithelium of the digestive tract and splanchnic mesoderm forms
the smooth muscle & connective tissue of the tract (except: mouth, terminal anal canal &
some salivary glands are lined by ectoderm)
• in regard to the adult:
- foregut becomes pharynx, esophagus, stomach & cranial duodenum; also, liver & pancreas
- midgut becomes small intestine, cecum, ascending colon, and part of the transverse colon
- hindgut becomes transverse & descending colon and the cloaca, the latter forms rectum
and most of the anal canal
(Note: In the adult abdomen, derivatives of the foregut, midgut, and hindgut are structures
supplied by celiac, cranial mesenteric, and caudal mesenteric arteries, respectively)

Pharynx — becomes a common respiratory & digestive chamber in the fetus and adult
• initially the pharynx is blind-ended, bounded cranially by the oropharyngeal membrane which
degenerates early (allowing pharynx to communicate with oral & nasal cavities)
• although the adult pharynx has a smooth wall, pharyngeal pouches appear during development;
the pouches give rise to a number of adult structures (e.g., parathyroid glands)
• two derivatives of pharyngeal pouches that retain continuity with the pharyngeal cavity are:
auditory tube & fossa of the palatine tonsil; the first pharyngeal pouch expands to form
the middle ear and its hollow connection becomes adult auditory tube
• a single thyroid gland originates from the floor of the pharynx (the gland is bi-lobed in the dog)
• a midline evagination of the floor of the pharynx gives rise to the larynx, trachea and lungs

Esophagus — undergoes principally elongation during development
• develops from foregut endoderm caudal to the pharynx
• skeletal muscle found in the wall of the esophagus & pharynx is derived from somites that migrate
into pharyngeal arches (IV-VI) and receive innervation from the vagus nerve
Note: Esophageal wall contains skeletal muscle throughout its length (dog, ruminants), to the
level of the diaphragm (pig), to the level of the heart in the mid-thorax (cat, horse, human),

or not at all (avian).

Stomach (simple stomach) — non-ruminant domestic mammals have single-compartment stomachs
• the stomach develops from a foregut segment that undergoes the following changes:
- growth is more rapid dorsally than ventrally; thus, the tube becomes convex dorsally (the future
greater curvature) and concave ventrally (future lesser curvature)
- the long axis becomes oriented transversely as growth of the liver pushes the cranial end of the
stomach toward the left side (the future greater curvature faces caudally, and drops ventrally
when the stomach is filled)
- growth along the cranial aspect of the future greater curvature exceeds that along the caudal aspect,
producing a fundus region facing left
• endoderm, which forms the epithelial lining of the stomach, differentiates into regional cells types that
vary among species; splanchnic mesoderm forms smooth muscle in the stomach wall

Ruminant stomach — ruminant animals include cattle, sheep, goats, deer, elk, moose, etc.
• the ruminant stomach is a complex stomach, consisting of four compartments:
- three are fermentation compartments lined by stratified squamous epithelium
(rumen, reticulum, and omasum)
- one is a glandular compartment like the simple stomach (abomasum)
• development of the ruminant stomach is initially the same as the simple stomach, then:
- the rumen develops as a bi-lobed expansion of the fundus region
- the reticulum arises as a caudoventral pocket of the growing rumen
- the omasum originates as a bulge along the lesser curvature, and
- the abomasum develops from the rest of the stomach
• later in development the growing rumen becomes “flipped” caudally, so it comes to lay
on top of the abomasum with the reticulum positioned cranially

Intestinal tract  — consists of small intestine (duodenum, jejunum, ileum) and large intestine
(cecum, colon, rectum, and part of the anal canal)
• adult intestine is derived from foregut (descending duodenum) midgut (regions supplied by the
cranial mesenteric a.) and hindgut (regions supplied by the caudal mesenteric a.)
• the embryonic midgut (where the yolk sac attaches) undergoes elongation and the following:
- the caudal limb of the midgut loop develops a diverticulum, the future cecum
- the elongating midgut forms a loop that protrudes out of the embryo into the cavity
of the umbilical stalk (normal umbilical herniation)
- the loop rotates 360° around the cranial mesenteric artery (former right vitelline a.);
the rotation is clockwise as viewed dorsally (rotation is facilitated by yolk sac detachment and
elongation of the cranial limb of the loop);
- the loop returns to the embryonic coelom (future abdominal cavity); failure to return
results in congenital umbilical hernia (omphalocoele)
• normally the yolk sac detaches from the midgut by atrophy of the connecting duct;
failure of the duct to atrophy can result in either a diverticulum of the jejunum, or
a fistulula (hollow channel) to the umbilicus, or a fibrous connection between the jejunum
and umbilicus (each of these can be a source of colic)
• the intestinal tract and esophagus normally become temporarily atretic (occluded lumen)
during development as a result of epithelial proliferation; recanalization occurs by
formation of vacuoles that coalesce to create the ultimate lumen. Persistent atresia
(failure to re-canalize) or stenosis (narrow lumen) is a congenital anomaly that can occur
at localized sites along the intestines.

Ascending colon modifications — modifications in ungulates to enhance fermentation
• initially, a loop forms in the ascending colon
• in pig and ruminants, the loop coils (forming a coiled or spiral colon)
• in the equine, the loop enlarges and bends on itself; also, the proximal colon becomes
incorporated within the base of the enlarged cecum

Cloaca — a common chamber for digestive, urinary, and genital tracts
• the hindgut terminates in a cloaca (temporary in mammals; permanent in birds, reptiles & amphibians)
• the caudal cloacal wall, the cloacal membrane, is formed by endoderm apposed to surface ectoderm
• the allantois grows out of the hindgut at the cranial end of the cloaca
• a urorectal septum divides the cloaca and cloacal membrane into dorsal (digestive) and ventral
(urogenital) chambers; the latter is connected to the allantois (the urinary bladder develops
from the urogenital chamber and allantois)

Rectum — the adult rectum extends from the pelvic inlet to the anal canal
• the dorsal chamber of the divided cloaca is continuous with the hindgut and becomes rectum
and most of the anal canal
• splanchnic mesoderm gives rise to rectal smooth muscle

Anal canal — the adult anal canal is enclosed within the striated external anal sphincter
• caudally, the urorectal septum divides the cloacal membrane into an anal membrane dorsally and
a urogenital membrane ventrally (subsequently, both membranes undergo degeneration)
• the cranial part of the anal canal (most of the canal) is formed along with the rectum;
this part of the anal canal is lined by a mucosal epithelium derived from endoderm
• the caudal part of the anal canal (caudal to the adult anocutaneous line) is lined by
stratified squamous epithelium; it forms as follows:
- tissue surrounding the anal membrane grows outward, creating a proctodeum lined by ectoderm
- the proctodeum is incorporated into the anal canal when the anal membrane degenerates
(atresia ani, an intact anal membrane, is a possible congenital anomaly)
- in carnivores, lateral diverticula of proctodeum ectoderm become adult anal sacs

Liver — the embryonic liver is a source of blood cells after blood stem-cells migrate into it
• the liver arises as an hepatic diverticulum of endoderm, ventrally from the region of foregut
that will become descending duodenum
• the hepatic diverticulum expands into the ventral mesogastrium; liver divides ventral mesogastrium
into lesser omentum and falciform & coronary ligaments. The hepatic diverticulum temporarily
pernetrates the septum transversum which contribute connective tissue to the liver
• the hepatic diverticulum gives rise to multiple buds that become: hepatic ducts,
the cystic duct, and the pancreatic duct
- continued growth and branching of hepatic duct primordia form lobes of the liver
- a gallbladder develops at the end of the cystic duct
- the initial hepatic diverticulum from which the various buds arose becomes the
major duodenal papilla and bile duct
Note: Although the hepatic diverticulum originates ventrally from the duodenal wall, differential
growth of the duodenal wall results in the adult bile duct and pancreatic duct entering the
major duodenal papilla dorsally in the duodenum.

Pancreas — a source of digestive enzymes and an endocrine gland
• the pancreas originates as two separate endoderm diverticula which elongate, branch, and
then forms acini in typical glandular fashion (some endoderm cells form endocrine islet cells)
• one diverticulum arises ventrally as a bud of the hepatic diverticulum; it forms the pancreatic duct
and right lobe of the pancreas
• the other diverticulum arises dorsally from the duodenum (minor duodenal papilla); it grows into the
dorsal mesogastrium and becomes accessory pancreatic duct and left lobe of the pancreas
• as the right and left pancreatic lobes cross one another during stomach rotation, they fuse to form the
body of the pancreas; also, the ducts anastomose to form a common duct system
Note: One of the two terminal pancreatic ducts will be smaller that the other and may even atrophy.
Which one become smaller or absent depends on the species. In dogs, the accessory pancreatic
duct is larger; in cats, only 20% have an acessory pancretic duct (& minor duodenal papilla).

Mesenteries — are formed by splanchnic mesoderm during creation of the embryonic gut as the
embryo assumes a tubular shape
• caudal to the pharynx dorsal and ventral “mesenteries” of the esophagus persist and become
mediastinum in the thorax
• in the abdomen, the dorsal mesentery is continuous, but the ventral mesentery is not present caudal
to the stomach and cranial to the cloaca
• the abdominal dorsal mesentery becomes: greater omentum, mesoduodenum, mesentery
(mesojejunum and mesoileum), mesocolon, and mesorectum
• the original dorsal mesogastrium elongates greatly as it forms the greater omentum;
the left lobe of the pancreas and the spleen develop within the greater omentum
• the ventral mesogastrium, in which the liver develops, becomes the lesser omentum and
coronary and falciform ligaments of the liver
• when the midgut elongates and rotates around the cranial mesenteric artery, portions of the
mesojejunum and mesoileum come into contact near the dorsal body wall and fuse,
forming the root of the mesentery
• caudally, ventral mesentery of the cloaca becomes median ligament of the urinary bladder


Avian — the avian digestive tract is different from the various mammalian versions
• the avian digestive tract features:
- a crop, which develops as a diverticulum of the esophagus;
- a two-compartment stomach:
1] proventriculus (glandular stomach) and
2] ventriculus or gizzard (stratified squamous epithelium and heavy muscles for grinding);
- a pair of ceca;
- a cloaca which opens externally by means of a vent