Overview of Events – 4^th week

• The head fold brings the heart into ventral location
• Tail fold aids formation of future umbilicus in ventral location
• Lateral fold forms tubular organization of embryo
• resulting in C-shaped embryo floating in amnion with connecting stalk on ventral side
• Lateral folds create endodermal tube, or gut tube
• endoderm Þ epithelial lining of gut
• endoderm Þ forgut, midgut, hindgut

• All 3 folds (head, tail, lateral) occur simultaneously ~day 22 and result in C-shaped, tubular embryo (Fig 1, 2, 3)
• Oropharyngeal membrane Þ oral opening
• Cloacal plate Þ anal opening
• As head folds; septum transversum, heart, pericardial coelom and oropharyngeal memb move to ventral surface
• part of yolk sac forms the foregut
• septum transversum Þ central tendon of diaphragm
• As tail folds:
• part of yolk sac Þ hindgut
• allantois moves ventrally Þ umbilical cord
• Folding of lateral sides (somatopleure) creates midgut
• endoderm and yolk sac Þ gut tube
• amnion completely surrounds the embryo
• connecting stalk, yolk sac remnant, and allantois Þ umbilical cord
• Intra-embryonic coelom Þ pericardial space, pleural space, peritoneal space
• Splanchnopleure Þ gut wall
• Somatopleure Þ body wall

Endodermal Derivatives ~ 30 days

• Endoderm Þ epithelial lining of digestive tract, its glands, liver, pancreas
• Splanchnic mesenchyme Þ muscular and connective tissue parts of the digestive tract
• Gut tube divided into 3 sections according to the arteries which feed each section