External: The body is more of a unified cuboidal mass, and both cervical and lumbar flexures are indicated; the limbs are longer, the digital plate of the hand is definitely notched, the elbow region is usually discernible, and toe rays can be identified in some specimens; eyelid folds are present in the more advanced embryos; a distinct tip of the nose can be seen in profile; auricular hillocks are being transformed into specific parts of the external

Internal: In the heart, septum secundum and the associated foramen ovale are appearing; the membra-nous part of the interventricular septum is beginning to take form; the vomeronasal organ is represented by a groove; choanae develop; some subsegmental buds develop in the bronchial tree; collecting tubules develop from the calices; testicular cords may begin to appear in the male gonad; the paramesonephric duct grows rapidly down through the mesonephros; 1-3 semicircular ducts are present in the internal ear.

• The body of the embryo is a more unified cuboidal mass than in earlier groups, and there is some indication of both lumbar and cervical flexures.
• The extremities are somewhat longer; the digital rays of the hand are definitely notched.
• The elbow region is usually discernable, and the foot plate and toe rays can be seen in some specimens.
• Eyelid folds are present in older specimens, and the pigmented retina is partly covered by white opaque condensed scleral masses, particularly above and on the lateral side of the retina.
• A distinct tip of the nose can be seen in profile.
• Auricular hillocks are being transformed into definite parts of the external ear.
• Ovulation age: 44-48 days

Question: What is the overall shape of the embryo at this point?

Developmental Stages in Human Embryos by Ronan O'Rahilly and Fabiola Müller.  Published by Carnegie Institution of Washington, Publication 637. 1987. Stage 18 (PDF)

Stage 17

Stage 19