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Embryology · General embryology: weeks 1 to 8

Embryonic folding: head, tail and lateral folds !

  • Process

In week four the flat disc bends over on itself and becomes a cylinder-shaped body with the gut inside.

Updated 2026-11-01

See it in 3D: Rectus abdominis

What it is

In week 4 the embryonic disc grows faster than the yolk sac, mainly because of the neural tube and somites, and folds in two directions. Cephalocaudal folding moves the heart and septum transversum ventrally and forms the foregut and hindgut; lateral folding closes the trunk wall and turns the endoderm into a tube. At the end the embryo sits inside the amnion, linked to the outside only by the umbilical cord.

Key facts

  • Week 4: cephalocaudal and lateral folding take place.
  • Head fold: the cardiogenic area and septum transversum move to the ventral chest region; the foregut forms.
  • Tail fold: the connecting stalk and allantois move ventrally; the hindgut and cloaca form.
  • Lateral folds: the body walls meet in the ventral midline, except at the umbilicus.
  • The midgut stays linked to the yolk sac by the vitelline (omphalomesenteric) duct.
  • The intraembryonic coelom ends up inside the body and will form the pericardial, pleural and peritoneal cavities.

Step by step

  1. The neural tube and somites grow and the disc bulges into the amniotic cavity.
  2. The head end bends ventrally: the heart and buccopharyngeal membrane end up on the ventral side.
  3. The tail end bends: the cloacal membrane and allantois move ventrally.
  4. The lateral edges roll ventrally and meet in the midline.
  5. The endoderm becomes the gut tube: foregut, midgut and hindgut.
  6. The amnion surrounds the embryo; the connecting stalk and vitelline duct come together in the umbilical cord.

Clinical notes

  • Gastroschisis: an abdominal wall defect, almost always right of the umbilicus, with bowel exposed and no sac; attributed to abnormal body wall closure.
  • Ectopia cordis: the heart lies outside the chest because the ventral chest wall fails to close.
  • Bladder and cloacal exstrophy: the infraumbilical body wall fails to close.
  • Omphalocele is different: the herniated midgut fails to return to the abdomen and stays covered by amnion.
  • Meckel diverticulum: a remnant of the vitelline duct that should have closed off.

Test yourself

Think of the answer, then tap the question to see it.

In which week does the embryo fold?

In week 4 after fertilization.

Which structures does the head fold carry ventrally?

The cardiogenic area (heart), the septum transversum and the buccopharyngeal membrane; it also forms the foregut.

Which abdominal wall defect is linked to abnormal body wall closure and has no sac?

Gastroschisis, almost always to the right of the umbilicus.

Which structure links the midgut to the yolk sac after folding?

The vitelline (omphalomesenteric) duct; if part of it persists, it forms a Meckel diverticulum.

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See it in 3D

The structures of Durgical's 3D anatomy where it is found.

← All of embryology

Educational content on embryonic development; not a guide to prenatal diagnosis or to medicines in pregnancy and no substitute for a health professional's advice. No doses are given.

Sources: Langman's Medical Embryology, 13th ed., ch. 7: The Gut Tube and the Body Cavities (Wolters Kluwer, 2015); Gastroschisis | Birth Defects | CDC (Centers for Disease Control and Prevention, 2026); Fetal development: MedlinePlus Medical Encyclopedia (MedlinePlus (U.S. National Library of Medicine), 2026).

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Representative model, for educational purposes only.