External: The trunk has begun to elongate and straighten slightly, with the result that the head no longer forms a right angle with the line of the back of the embryo. The limbs extend nearly directly forward; The toe rays are more prominent, but interdigital notches have not yet appeared in the rim of the foot plate. 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.

• Length 17 - 20 mm
• Trunk beginning to straighten out and cervical angle less acute.
• Coalescence of parts of branch region has altered appearances of external ear, and the hillocks are less conspicuous.
• Axes of arm are almost at right angles to the dorsal line of the body of the embryo.
• Various parts in leg and arm can be identified.
• Toe rays are more prominent.
• Ovulation age: 48 - 51 days

Question: What has altered the appearance of the external ear?

Developmental Stages in Human Embryos by Ronan O'Rahilly and Fabiola Müller.  Published by Carnegie Institution of Washington, Publication 637. 1987. Stage 19 (PDF)

Stage 18

Stage 20 Stage 19 Internal