As indicated by phrenic nerve outgrowth, the diaphragm originates at the cervical level and descends to the base of the thorax. (A–B) Phrenic nerve, labeled via neurofilament antibody, emerges at E11.25 from the spinal cord at C3–C5 level and extends medially to the developing diaphragm located at C5 level (blue arrowheads and dotted line). (C–F) By E11.5 phrenic nerve axon tips and developing diaphragm (blue arrowheads) have descended to C8 level and by E12.5 to T5 level. (D) Note left phrenic nerve consistently lags behind right nerve. vt, ventricle. Scale bar A–D = 200um, E = 100um, F = 200um.