The Stages of Human Embryonic Development

Stage 17 embryos have a greatest length of 11 to 14 mm and an estimated post-fertilization age of 41 days. Embryos are becoming less 'C'-shaped as the main axis of the trunk becomes straighter. In some stage 17 embryos a slight lordotic curvature may be seen in the lumbar region. Other characteristic external features include the presence of all the auricular hillocks, definite digital rays in the hand plate and a rounded digital foot plate.

Internally, the palate is beginning to develop, auditory ossicles are apparent, and the olfactory bulb begins to appear. Chondrification starts in some of the vertebral bodies, the humerus and the radius. The dorsal and ventral parts of the pancreas are fused and the vermiform appendix is visible for the first time.