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Chick embryo rhombomere neural crest cells and their musculoskeletal “packets.” (A) 2-day chick embryo showing the contribution of the rhombomere neural crest cells to the pharyngeal arches. (Most of the neural crest cells from r3 and r5 undergo apoptosis, while the rest of these cells contribute to the larger population of r4 neural crest cells.) (B) 10-day embryo showing the bones of the upper and lower jaws, tongue skeleton, and middle ear derived from the rhombomeric crest cells. The muscles derived from r4 are attached to bones from the same rhombomere, and the part of the braincase attached to the r4-derived jaw-opening muscle is also derived from r 4. (Other muscles have been omitted for clarity.) (After Ahlberg 1997.)