External: The head is relatively larger than previously; the main axis of the trunk has become straighter, and a slight indication of a lumbar curvature may be found; the nasal pit is further medial and is directed ventrally so that the nostril is not visible in profile views, and nasofrontal grooves are distinct; the full complement of auricular hillocks is present on the mandibular and hyoid arches; the hand plate exhibits definite digital rays, and the foot has acquired a rounded digital plate; surface elevations of individual somites are becoming limited mostly to the lumbosacral region.
Internal: In the heart, foramen secundum and semi-lunar cusps have appeared (stages 15-17), and fora-men primum is being obliterated and the atrioventricular cushions are fusing (stages 16-18); the beginnings of the palate are appearing, the dorsal and ventral pancreas are fused, and the vermiform appendix is distinguishable; the bronchial tree shows segmental buds; the mesonephros is functional, calices are developing, and the urogenital sinus presents two divisions; chondrification begins in some of the vertebral centra and in the humerus and radius; the future olfactory bulb is indicated; the retinal fissure is largely closed and the lens cavity is becoming crescentic; semicircular ducts are imminent but none is yet present; the auditory ossicles are defined.
Developmental Stages in Human Embryos by Ronan O'Rahilly and Fabiola Müller.
Published by Carnegie Institution of Washington, Publication 637. 1987.
Stage 17 (PDF 1.1 MB)
QUESTION: What is present on the mandibular and hyoid bars?