At approximately day 1

Blood flow begins and heart begins to beat.

Reference

Moore KL, Persaud TVN. 1998. The Developing Human: Clinically Oriented Embryology. 6th ed. Philadelphia: WB Saunders Co.

At approximately day 1

The pharyngeal arches, supplied by the aortic arches (arteries), transform into the adult arterial arrangement by week 8.

Duration: 4 weeks

Reference

Moore KL, Persaud TVN. 1998. The Developing Human: Clinically Oriented Embryology. 6th ed. Philadelphia: WB Saunders Co.

At approximately day 1

Three paired primordial veins develop into adult venous patterns by week 9.

Duration: 5 weeks

Reference

Moore KL, Persaud TVN. 1998. The Developing Human: Clinically Oriented Embryology. 6th ed. Philadelphia: WB Saunders Co.

At approximately day 1

Two rudimentary and transitory sets of kidneys (pronephros and mesonephros) appear; pronephri degenerate within a few days.

Reference

Moore KL, Persaud TVN. 1998. The Developing Human: Clinically Oriented Embryology. 6th ed. Philadelphia: WB Saunders Co.

At approximately day 1

Heart tube lengthens and bends, forming the cardiac loop.

Reference

Sadler TW. 2004. Langman’s Medical Embryology. Ninth Edition. Maryland: Lippincott Williams & Wilkins.

At approximately day 1

Stomach becomes apparent as the foregut expands slightly and begins to elongate rapidly.

Reference

Larsen WJ. 1997. Human Embryology. New York: Churchill Livingstone Inc.

At approximately day 1

The rudimentary lung bud appears as an outpouching of the endodermal foregut.

Reference

Larsen WJ. 1997. Human Embryology. New York: Churchill Livingstone Inc.

At approximately day 1

The lung bud bifurcates into right and left primary bronchial buds.

Duration: 1 week

Reference

Larsen WJ. 1997. Human Embryology. New York: Churchill Livingstone Inc.

At approximately day 1

Esophagus can be identified; it elongates relatively rapidly compared to the whole fetus.

Reference [PubMed Link]

Montgomery RK, Mulberg AE, Grand RJ. 1999. Development of the human gastrointestinal tract: twenty years of progress. Gastroenterology 116(3):702-731.

At approximately day 4

Heart becomes partitioned into four chambers by week 7.5.

Duration: 3 weeks

Reference

Moore KL, Persaud TVN. 1998. The Developing Human: Clinically Oriented Embryology. 6th ed. Philadelphia: WB Saunders Co.

At approximately day 7

The atrial, ventricular, and conotruncal septa form.

Duration: 2 weeks

Reference [PubMed Link]

Sadler TW. 2000. Susceptible periods during embryogenesis of the heart and endocrine glands. Environ Health Perspect 108 (suppl 3):555-561.