The gastrointestinal tract is one of the earliest organ systems to form in the human embryo; it develops as a diverticulum off the yolk sac. The most primitive fishes exhibit a tubular digestive tract, functioning proximally in transport and distally in nutrient breakdown and absorption. The expansion of this early gut tube into an intricate digestive system involving accessory organs and intestinal rotation reflects the evolution of more complex dietary habits. Several features of gastrointestinal development also demonstrate the transition from water to land organisms. A recent article reviewed head and neck embryology with links to evolutionary anatomy, to provide context to the morphology of primitive structures, and explain complexities in head development [1]. Similarly, we believe the study of digestive comparative anatomy puts the embryology and modern anatomy into perspective, and facilitates a deeper comprehension of the human gastrointestinal tract.

Feeding to obtain nutrients, such as pre-synthesized amino acids, carbohydrates, and smaller biomolecules is an essential process common to all multicellular organisms. It is crucial to absorb organic molecules and water, and subsequently break down the materials to be used in various cellular processes. There are three types of multicellular organisms: fungi, plants, and animals. Fungi are composed of filaments that invade decomposing materials such as fruits or carcasses. They absorb the nutrients at the site and incorporate them into their own cells [2]. Plants rely on solar energy, which is converted to usable nutrients via chlorophyll molecules. They also obtain minerals and water from their roots that penetrate the surrounding soil [3] [4]. Animals ingest food, which is subsequently processed by a tubular digestive system. The food particles are broken down mechanically and chemically by digestive enzymes and intestinal bacteria. Both absorption and digestion take place in the intestines, and wasted bacteria and unused food material is excreted.

Comparative anatomy and evolutionary biology provide context to the development of early digestive structures, however these courses have been lost in the pre-medical and medical curricula [5]. A key concept in anatomy involves relating form to function; comparative anatomy demonstrates how structural adaptions can meet environmental or changing physiologic needs. A current recommendation by the American Association of Medical Colleges is to incorporate evolutionary biology in medical education [6]. Many articles argue the importance of Darwinian medicine in training future physicians [7] [8][9]. Evolutionary biology teaches medicine at the molecular level, for example demonstrating the virulence of viruses via genetic mutations, and at the epidemiological level, such as relating malarial resistance to the sickle-cell trait carrier by natural selection [10] [11]. Studies in North America and in the UK showed a need for evolutionary biology to be incorporated into standard medical education courses [12][13].

In this article, we summarize the key embryological events in human gastrointestinal development, while providing connections to comparative digestive anatomy. The evolutionary history of the gut tube puts the modern anatomy and development into context, and enriches the medical student knowledge of digestive adaptations by relating structure to function. We also describe important congenital anomalies of the gastrointestinal tract; an understanding of these anomalies helps reinforce knowledge of the normal embryology. We believe this comprehensive review of gastrointestinal embryology and comparative anatomy will enable medical students to better understand gut development and adaptations to various evolutionary ecological conditions.

During third to fourth week of development, the embryo folds and the innermost endodermal layer forms the gut tube, which communicates ventrally with the yolk sac (Figure 1). The gut forms as a diverticulum off of the yolk sac. The gut tube is divided regionally into the foregut, which terminates cranially in the oropharyngeal membrane, midgut, and the hindgut, which terminates caudally in the cloacal membrane. The oropharyngeal and cloacal membranes are the only two regions in the body where ectoderm directly contacts endoderm. These membranes eventually rupture, making the gut tube a patent pathway proximally and distally. As folding and gut tube lengthening continue, the ventral connection between the midgut and yolk sac narrows and elongates to form the vitelline duct. The endodermal gut tube is surrounded by lateral plate splanchnic mesoderm, which forms the lamina propria, submucosal, and mucosal layers of the gastrointestinal tract. The splanchnic mesoderm also forms the smooth muscles of the intestines and of the lower esophagus. Neural crest cells migrate to the developing gut tube and form the enteric nervous system. The autonomic (parasympathetic and sympathetic) innervation is also formed, largely by neural crest, and connects the gastrointestinal system to the brain. Therefore, the gut tube is controlled by two nervous systems [14].

The gut tube is surrounded by the splanchnopleure, which ultimately forms the visceral peritoneum. A broad layer of mesenchyme initially attaches the abdominal gut tube to the body wall. This attachment subsequently thins and forms the dorsal mesentery. The gut tube becomes suspended in the peritoneal cavity, and the viscera therefore are intraperitoneal. These intraperitoneal structures are suspended in mesenteries, but are still mobile. Retroperitoneal structures are organs deep to the abdomen, or posterior to the peritoneal cavity. Primarily retroperitoneal structures originate behind the peritoneal cavity, and are never suspended; these include the kidneys, adrenals, ureters, and bladder. Some regions of the gut tube that develop initially within the peritoneal cavity are rotated and fuse to the posterior body wall; these are termed "secondarily retroperitoneal" and include the duodenum, pancreas, ascending colon, and descending colon [15].

Evolutionary link
In primitive fishes and amphibians, the developing gut tube was comprised of a single pouch of endoderm with cells rich in yolk material. The richness in yolk preceded the origin of a true yolk sac. In more advanced groups, endoderm grows to surround the yolk, enclosing it in a yolk sac. The yolk is progressively digested during development, and the connection between the yolk sac and growing endoderm thins, eventually forming a stalk, or the vitelline duct [16].

The regional delineations of the foregut, midgut, and hindgut are not common to all species. In the more primitive form, such as in sharks and certain fishes, there is minimal anatomical distinction between the small and large intestines, and often there is no discrete out pouching to form the stomach. The pylorus, however, which delineates the transition from stomach to intestines, is common to most vertebrates. Primitively, the gut tube regions proximal to the pyloric constriction functioned in food transport from the mouth, and regions distal to the pylorus were responsible for digestion and absorption. The foregut evolves to play a larger role in nutrient breakdown and food storage, and the stomach becomes pivotal for chemical and mechanical digestion [17].

Initially, the pancreas, spleen, stomach, and liver are aligned dorsoventrally in the median plane, and are connected by ligamentous or mesenteric attachments. The pancreas and spleen are joined within the dorsal mesogastrium, the spleen and stomach are connected by the gastrolienal ligament, the stomach and liver are joined by the hepatogastric ligament, and the liver connects to the ventral wall with the falciform ligament (Figure 2). This line of organs divides the abdominal cavity into two initially equal areas. However, as abdominal organs enlarge and rotate, the dorsal mesogastrium is repositioned and creates a small space behind the stomach termed the lesser peritoneal sac. The remainder of the peritoneal cavity, comprising the majority of the abdomen becomes the greater peritoneal sac; the two spaces are connected via the epiploic foramen. The epiploic foramen represents the central point of embryonic rotation [18].

The stomach attaches to the posterior body wall via the dorsal mesogastrium. The double-layered dorsal mesogastrium extends caudally during rotation of the stomach, and fuses with the double-layered mesentery of the transverse colon to create the greater omentum. The greater omentum is thus composed of four layers of visceral peritoneum. The lesser omentum extends from the stomach and duodenum to the liver and includes the hepatogastric ligament, which joins the liver with the stomach, and the hepatoduodenal ligament, which connects the liver to the duodenum. The hepatoduodenal ligament contains the portal triad (bile duct, portal vein, and hepatic artery) [19].

Evolutionary link
The rotation of the stomach and concurrent pulling of the dorsal mesogastrium to form the greater omentum likely began with the evolution of jawed vertebrates. In reptiles and birds, the lungs are separated from the peritoneal (abdominal) cavity by an oblique septum, thus allowing for a more extensive respiratory system to develop. In mammals, this is accomplished by the development of the diaphragm from the embryonic transverse septum, which not only separates the thoracic and abdominal cavities, but also functions in drawing air into the lungs during respiration. The liver of vertebrates grows into the transverse septum, and as the liver wall expands, it spreads the septum to form its serosal covering, as well as the coronary ligament cranially, and the falciform ligament ventrally [20].

The esophagus is the cranial-most structure formed from the foregut. The respiratory diverticulum forms on the ventral surface of the developing esophagus; this will further differentiate to form the trachea, bronchi, and lungs. The respiratory diverticulum is separated from the cranial foregut by the tracheoesophageal septum. The esophagus is initially a short structure that progressively grows to span the length of the thoracic cavity. The inner wall of the esophagus is comprised of endoderm. The musculature of the upper esophagus is striated muscle, derived from branchial arch mesoderm, and the musculature of the lower esophagus is smooth muscle, derived from splanchnic mesoderm [21].

Evolutionary link
In primitive fishes, the esophagus, and majority of foregut in general, is reduced to a simple tube connecting the mouth and pharynx to the intestines. The inner surface is ciliated, facilitating transport in water (Figure 3). In many more advanced fishes, the stomach develops, and a sphincter is often present to control passageway from the esophagus into the stomach. The evolution of a discrete esophagus coincides with the development of land vertebrates. This occurs with the reduction of gills and therefore the pharynx, allowing the esophagus to elongate and eventually span the length of the neck and thoracic cavity. In land mammals, the ciliated epithelium is often lost, and striated and smooth musculature surrounding the esophageal wall appears. Therefore, ciliary transport, which is better adapted for particle transport in water, is replaced by peristaltic muscular contractions, allowing for voluntary control of swallowing on land. The development of esophageal musculature also allows for reverse transport from the stomach to the mouth, which serves as a protective mechanism to expel potentially toxic substances. This also facilitates regurgitation of partially digested food material, a behavior exhibited by ruminants for specialized digestion and birds for feeding their young. Birds have an additional esophageal adaptation where a distensible sac is present, termed the crop, which is a temporary site for food storage before regurgitation (Figure 3) [17].

The stomach forms as a dilatation of the foregut. The developing stomach initially connects to the ventral body wall via the septum transversum. This eventually thins and forms the ventral mesentery. Around fifth week, the stomach wall grows asymmetrically; the dorsal wall grows faster and creates the greater curvature, and the ventral wall forms the lesser curvature. During weeks seven and eight, the stomach rotates 90 degrees around a longitudinal axis, bringing the greater curvature to the left and the lesser curvature to the right (Figure 4). This rotation creates the C-shaped loop characteristic of the duodenum, and moves it towards the dorsal body wall, with which it fuses to form the lesser sac. The duodenum delineates the caudal portion of the foregut and cranial portion of the midgut, and is thus supplied by both the celiac and superior mesenteric arteries [22].

Evolutionary link
In early fishes, there is no muscular dilatation of the foregut that characterizes a distinct stomach. In these species, food particles are passed directly into the intestines, without delay in transport or true digestion. The evolution of the stomach allows for storage of food before passage into the intestines, thus permitting more complex feeding habits, such as the ingestion of large quantities of material that would otherwise overwhelm the intestines. Storage of food in the stomach and selective transport through the pylorus at neurally regulated time intervals prevents overwhelming the intestines during large meals, and maximizes digestion and absorption of nutrients. Many bird species further exploit this function and store meals and subsequently regurgitate the partially digested material to feed their young. The stomach also develops mechanical and chemical digestive capabilities, likely with the evolution of jawed fishes that ingest larger meals at irregular intervals. Many variations of stomach shape exist in living vertebrates: the gizzard of birds allows for mechanical digestion using swallowed pebbles, largely reducing the need for teeth (Figure 3). The four-chambered stomach of mammalian ruminants (Figure 4) allows for regurgitation of cud and more extensive breakdown of plant nutrients; the first three chambers secrete liquid and house microorganisms that help breakdown cellulose, and the fourth chamber contains the regions of a "typical" mammalian stomach (cardiac, fundic, and pyloric). Although the stomach shape is highly variable, the majority of living species exhibit an asymmetry of the left and right stomach wall (greater and lesser curvatures) and a general curvature to the left, reflecting the overgrowth of the dorsal wall and rotation of the entire organ seen in human development [17].

The liver forms as a diverticulum from the foregut. Liver hepatoblasts differentiate into the hepatocytes, bile canaliculi, and hepatic ducts. The connection between the liver and duodenum narrows during development, forming the bile duct. The liver gains its hematopoietic function via migrating stem cells from the yolk sac, which is the initial site of hematopoiesis in the embryo. The liver loses its hematopoietic function shortly after the birth of the fetus, at which time the bone marrow becomes the primary site of red and white blood cell production. The liver is supplied by the vitelline vein, which brings nutrient rich blood from the yolk sac. The vitelline vein forms the adult portal vein, which drains the digestive tract. The majority of the liver is endodermal in origin, with mesodermal stromal support from the septum transversum and splanchnic mesoderm. The liver parenchyma is penetrated by branches of the vitelline and umbilical veins, forming the hepatic sinusoids. The gallbladder and cystic duct develop from the cystic diverticulum, which forms from the bile duct. The cystic duct and bile duct merge to form the common bile duct, which opens into the major duodenal papilla [21].

Evolutionary link
The liver is a structure common to all vertebrates, and it always occupies a considerable space in the ventral abdominal cavity. The mammalian liver closely resembles a glandular gut diverticulum seen in early taxa such as Amphioxus, suggesting its most primitive function was in digestive enzyme production. This is reflected in its drainage into the duodenum, allowing for direct transport of digestive material such as bile into the intestinal lumen. The liver also gains an extensive metabolic role, and it produces, alters, and stores fats, proteins, sugars, vitamins, and many other materials. This systemically crucial role of the liver is supported by its extensive venous supply, where all gastrointestinal organs first drain into the hepatic portal system before ultimately reaching the heart (see "vasculature" below). The presence of the gallbladder and drainage of the cystic duct is variable. Interestingly, among the relatively closely related ungulates, cervids lack a gallbladder, bovids possess a gallbladder, and giraffids possess a gallbladder during fetal life but lack one as adults [16] [23] [24][25].

Like the liver and gallbladder, the dorsal and ventral buds of the pancreas form as diverticula from the duodenum. The ventral bud forms the uncinate process and the caudal part of the head, and migrates posteriorly around the duodenum to fuse with the dorsal bud, which forms the cranial part of the head, as well as body, and tail of the pancreas (Figure 5). This migration occurs concurrently with the posterior rotation of the duodenum. The ductal systems of both buds fuse, and the ducts from the dorsal bud drain into the ventral ducts, creating the main pancreatic duct. This drains into the duodenum at the ampulla of Vater, along with the common bile duct. The pancreatic endoderm differentiates into cells that perform the many functions of the pancreas. The exocrine (acinar) cells function in digestion, the ductal cells function in transport, and the endocrine cells produce hormones such as insulin, glucagon, and somatostatin. The endocrine cells aggregate into islets, which become functional around the fifth month of development [14].

Evolutionary link
Many primitive fish species lack a discrete pancreas, but instead possess a mass of enzyme secreting cells aggregated at the proximal intestines near what will become the major duodenal papilla. Later in evolution, pancreatic tissues develop as an outgrowth of the bile duct, and still retain their position adjacent to the liver. Correspondingly, as the pancreas develops as a separate organ, its duct system is shared with the drainage of bile from the liver, and the two systems drain into a common opening in the duodenum, which marks the original location of pancreatic cells seen in primitive organisms. In fishes, the dorsal and ventral buds form separate structures with independent drainage. This resembles the occasional condition seen in humans, where drainage of the dorsal and ventral buds does not fuse, forming the main and accessory pancreatic ducts respectively [20].

Around week 5, the midgut undergoes rapid elongation, and forms a hairpin loop around the superior mesenteric artery termed the primary intestinal loop (Figure 6). The cranial limb of the loop creates the jejunum, and parts of the duodenum and ileum, and the caudal limb of the loop develops into the cecum, appendix, ascending colon, and parts of the ileum and transverse colon. As the liver rapidly expands in size, the abdominal cavity becomes temporarily too small to accommodate the proliferating intestinal loop, and the intestinal loop thus herniates into the umbilicus. The midgut rotates around the superior mesenteric artery 90 degrees counterclockwise in the umbilicus, and another 180 degrees as the elongated intestines fall back into the abdomen around week 10. The jejunum and ileum also coil and loop separately from the colon. The final midgut rotates an additional 180 degrees counterclockwise, placing the cecum at the right lower quadrant. The midgut, in total, rotates 270 degrees. The ascending and descending colon contact the dorsal body wall upon their return into the abdominal cavity, and eventually fuse, becoming secondarily retroperitoneal [15].

Evolutionary link

Sharks, and some primitive fishes lengthen the intestinal surface area with spiral valves within the intestinal tube. The spiral folds of mucosa extend from one end of the intestine to the other. Vertebrates expand the absorptive capacity of the intestines by extending the mucosal surface into villi and crypts, thus increasing the surface area. In many vertebrates, the intestine itself is elongated and coiled, thus necessitating its herniation into the umbilical cord to accommodate for massive lengthening during development. In mammals and birds, this elongation and rotation can generate intestinal lengths 7–8 times the height of the entire body. The absolute length of the intestines is highly variable, and often relates both to body size and dietary habits. In frog tadpoles, which are largely herbivorous, the intestines are long and extensively coiled, allowing for better plant digestion, whereas the adult frogs after metamorphosis possess shortened intestines, as they are mainly carnivorous and don't require extensive nutrient breakdown. Herbivorous species generally have more elongated intestines, allowing for the ingestion of cellulose plant material, which necessitates more extensive digestion and larger surfaces for absorption [20].

The distal-most portion of the embryonic hindgut as well as the developing urinary and reproductive tracts is continuous with the cloaca (Figure 7). The cloaca forms the anorectal canal posteriorly and urogenital sinus (future bladder, urethra, prostate, and vestibule of the vagina) anteriorly. The anus and urogenital sinus become separated by the urorectal septum, which forms the perineal body [21].

The cloacal membrane is endodermal dorsally and ectodermal ventrally, with no mesoderm in between. The membrane ruptures, allowing the anal canal to open to the exterior, thus making the gastrointestinal tract a patent pathway. In the anus, ectodermal cells of the anal membrane proliferate and temporarily plug the lumen. The membrane obliterates around week 8, and its location is marked by irregular mucosal foldings, forming the pectinate line. Anything proximal to this is formed from the endoderm of the hindgut, and distal to the pectinate line is formed by the ectoderm of the cloaca. The distal third of the anal canal is referred to as the proctodeum. As the distal and proximal anal canal have different embryologic origins, they are supplied by separate vasculature and innervation (Table 1) [14].

Evolutionary link
The colon of most mammals is larger in diameter than the small intestines, and contains characteristic intestinal outpouching and longitudinal muscle fibers. As aquatic organisms evolve to live on land, the colon undergoes substantial lengthening. One of the major functions of the colon is to reabsorb water, and as vertebrates become adapted for terrestrial life, longer large intestines facilitate better water retention. The general shape involves a right-sided ascending colon and left-sided descending colon. In primates, a transverse colon is present and extends the connection between the ascending and descending colon, thus increasing the surface area of haustrated intestines. Herbivores increase the length of the colon with additional coiling [26].

Several groups of animals, including amphibians, reptiles, and birds, maintain the distal connection between the urinary, genital, and gastrointestinal tracts as a cloaca. The formation of a cloaca at some point during development is common to all vertebrates, however in most mammals, this distal cavity is partitioned into the anorectal canal and urogenital sinus by the urorectal septum. This septum creates separate external openings for the gastrointestinal and genitourinary tracts. In monotremes, such as the platypus, the subdivision of the cloaca is partially completed, and a fold marks the delineation between but does not fully separate the distal intestines from the distal urinary and reproductive tracts [17].

The yolk sac is initially a large and major embryonic structure, and it contains its own blood supply arising from the dorsal (abdominal) aorta. The gastrointestinal tract and associated organs derive from the yolk sac, thus the vitelline arteries persist in the adult to supply the digestive organs. The vitelline arteries arise from the aorta caudal to the diaphragm, and form in the adult the celiac artery (foregut), superior mesenteric artery (midgut) and inferior mesenteric artery (hindgut). During early development, these arteries bring deoxygenated blood from the heart towards the yolk sac [22].

The embryo has three venous systems: the vitelline system, the umbilical system, and the cardinal system. The umbilical and vitelline veins bring blood from the placenta and yolk sac, respectively, into the liver and heart. The vitelline and umbilical systems contribute to the ultimate venous drainage of the developed gastrointestinal tract. The vitelline veins traverse the developing liver and create the liver sinusoids. Around the third month, the left vitelline vein regresses, and the right vitelline vein drains the entirety of the gastrointestinal tract and diverticula. The vitelline venous system creates the portion of the inferior vena cava between the liver and heart, portal vein, splenic vein, and the veins draining the gastrointestinal tract, including the superior mesenteric vein and inferior mesenteric vein (Figure 8) [14].

The umbilical veins drain oxygenated blood from the placenta to the embryonic heart. The right umbilical vein ultimately regresses, and the left umbilical vein persists as the ductus venosus. The ductus venosus is an anastomosis between the inferior vena cava and umbilical vein, allowing oxygenated blood to bypass the liver and drain directly into the right atrium. This channel is closed after birth, as the placenta no longer supplies oxygenated blood, and creates the ligamentum venosum [21].

Evolutionary link
In fishes and amphibians, the yolk sac does not have its own vasculature, and is instead supplied by systemic arteries and veins. In mammals, birds, and reptiles, however, the yolk sac evolves its separate drainage, thus forming the vitelline arteries and veins. As the liver develops into a massive abdominal structure, the vitelline veins are divided into smaller hepatic sinusoids as they traverse the liver towards the heart; all gut veins first converge in the liver, thereby forming the hepatic portal system common to all vertebrates. The development of the vitelline drainage system allows for the evolution of extensive hepatic metabolic functions, as all nutrients absorbed in the digestive tract are drained directly to the liver. In many fishes, blood from the liver is drained directly into the sinus venosus of the developing heart; in mammals, reptiles, amphibians, and birds, however, the vitelline system creates the portion of the inferior vena cava between the liver and heart, and thus hepatic blood flow drains first into this large composite vein [27].

Tracheoesophageal fistula
Failure of the tracheoesophageal septum to fully separate the proximal foregut from the developing respiratory tree. Several variations exist; the most common anomaly presents as a blind ending esophagus (esophageal atresia), and distal anomalous connection between the esophagus and trachea. During development, the blind ending esophagus prevents the normal swallowing of amniotic fluid, resulting in a dilated proximal esophagus, and polyhydramnios. This greatly increases the risk of aspiration after birth, and can present with drooling, choking, and cyanosis with feeding. This is component of "VACTERL" association, which is a group of congenital anomalies that commonly coexist, and are often seen with chromosomal disorders. The VACTERL association includes vertebral anomalies, anal atresia, cardiovascular anomalies, tracheoesophageal fistula, esophageal atresia, renal anomalies, and limb defects [28].

Pyloric stenosis
The pylorus of the developing stomach hypertrophies, causing gastric outlet obstruction. This creates an "olive" mass, created from the thickened pyloric circular muscle. After birth, this presents as non-bilious projectile vomiting; bile is absent since the constriction is proximal to the bile duct [29].

Annular pancreas
The pancreas develops as a bilobed ventral bud, rather than the typical dorsal and ventral buds. One lobe migrates posteriorly and the other migrates anteriorly, and the two buds join to form a ring around the duodenum. The buds compress the duodenum causing obstruction. This prevents the fetus from swallowing amniotic fluid during development, causing polyhydramnios, and leads to vomiting and abdominal distension after birth [30].

Intestinal malrotation
Rotation of the midgut is not complete by the 10th-12th week. The cecum is positioned adjacent to the duodenum, and is anomalously attached to the vasculature of the mesentery. This allows for torsion around the attachment, possibly leading to obstruction and necrosis. This presents with bilious vomiting, and on imaging exhibits an "apple peel" appearance of the jejunum [31].

Midgut volvulus
Fixation of the midgut to the body wall via fibrous bands, termed "Ladd's bands" due to intestinal malrotation. This can compress the vasculature supplying the intestines, leading to necrosis, or cause obstruction leading to bilious vomiting [32].

Meckel's diverticulum
Failure of the vitelline duct to fully regress, which typically occurs around weeks 5–8. This is a true diverticulum that involves all layers of the intestinal wall. Meckel's diverticulum can present as ectopic gastric or pancreatic tissue secreting acid or digestive enzymes, causing ulceration. After birth, this leads to painless bleeding (melena), right lower quadrant pain, peritonitis, or obstruction within the first two years of life. A variation of Meckel's diverticulum is an omphalomesenteric fistula, where the ileum maintains an open connection with the umbilicus. This would present as meconium leaking through the umbilicus after birth [33].

Umbilical hernia
Protrusion of bowel through the umbilical ring. This often regresses spontaneously by age 5 [34].

Omphalocele
Herniation of bowel through the umbilical ring, covered with a membranous sac. This is often seen with chromosomal abnormalities [35].

Gastroschisis
Herniation of the bowel through the weakness in the anterior abdominal wall, lateral to the umbilicus. The herniated bowel is not contained in a sac [35].

Imperforate anus
Dysfunction of the urorectal septum, which normally separates the cloaca into the urogenital sinus and anorectal canal. The rectum opens into the reproductive or urinary tract (as a fistula between the bladder, urethra, or vagina), or a low lesion where the rectum ends in a blind pouch. After birth, this presents as failure to pass meconium, with a "dimple" where the anus should be [36].

Hirschsprung's disease
Failure of the neural crest cells to migrate to the distal anus, thus creating an aganglionic segment of colon. This section is unable to relax and pass stool. At birth, this presents as obstruction and failure to pass meconium, and can lead to the condition termed megacolon [37].

Studying evolutionary anatomy helps put human embryology into perspective; the concepts help explain why humans develop the way we do, rather than just memorizing how. Comparative anatomy helps link form to function, and leads to an overall deeper understanding of the embryology and anatomy, and an appreciation for the complexity of other animals. Studying congenital anomalies helps reinforce embryological concepts, and learning these anomalies in their developmental context facilitates long-term retention of knowledge. We believe reviews of each system of embryology supplemented with the evolutionary anatomy and associated congenital anomalies will help students better understand human development.

We thank the Department of Anatomy and the medical students of New York Institute of Technology College of Osteopathic Medicine. We thank our collaborators Bennett Futterman, Brooke Littlefield, Michelle Annabi, Kristen Farraj, and Samantha White.

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