External: All four limb buds are usually visible; the optic vesicle is closed; the lens disc is generally not yet indented.

Internal: Septum primum and foramen primum appear in the heart; right and left lung buds are recognizable, and the trachea begins its development; retinal and lens discs appear; the endolymphatic appendage becomes distinguishable by the end of the stage.

External View	Internal View

• Length: 4 - 6 mm
• Pore of optic invagination closed.
• Lens vesicle not yet indented.
• Definite arm buds and beginning leg bud.
• Heart chambers distended.
• Ovulation age: 28 - 32 days

Question: What are some characteristics of the heart
chambers?

Developmental Stages in Human Embryos by Ronan O'Rahilly and Fabiola Müller.  Published by Carnegie Institution of Washington, Publication 637. 1987. Stage 13 (PDF)

Stage 12

Stage 14 Stage 13 Internal