At approximately day 1

Cells in the septum primum (which divides the primitive atrium) undergo apoptosis. Abnormal development leads to septal defects.

Duration: 1 week

Reference [PubMed Link]

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

At approximately day 1

Liver grows rapidly, accounting for 10% of the total weight of the fetus 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

Metanephric blastema develops from intermediate mesoderm.

Reference

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

At approximately day 1

Aortic and pulmonary semilunar valves form and are complete by week 10.

Duration: 5 weeks

Reference

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

At approximately day 1

Bicuspid (mitral) and tricuspid atrioventricular valves begin to develop, complete by week 12.

Duration: 7 weeks

Reference

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

At approximately day 1

The dorsal wall grows faster than the ventral wall, forming the greater and lesser curvatures of the stomach.

Reference

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

At approximately day 1

Presumptive ileum begins to elongate rapidly, becoming the primary intestinal loop.

Duration: 1 week

Reference

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

At approximately day 1

Between weeks 5 and 28 the bronchial buds branch 16 times to form the respiratory trees of the lungs.

Duration: 23 weeks

Reference

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