(A, Ai) Microinjection of CM-DiI into the head mesoderm (HM) of a skate embryo at S18. (B, Bi) Simultaneous labelling of the hindbrain neural tube (including premigratory cranial neural crest cells) with CM-DiI and lateral plate mesoderm (LPM) with SpDiOC18 in a S18 skate embryo. (C, Ci) LPM gives rise to chondrocytes within the skeleton of the pectoral fin and girdle, while mesoderm at the HM-LPM boundary and LPM give rise to chondrocytes within the gill arch skeleton – e.g. (D, Di) in the branchial rays of gill arch 2. (E) After double labelling of the LPM with SpDiOC18 and the neural tube with CM-DiI, as in (B) above, both (Ei) SpDiOC18- and (Eii) CM-DiI-labelled chondrocytes are recovered within the gill arch skeleton – for example, in the ceratobranchial of gill arch 4 – demonstrating the dual mesodermal and neural crest origin of these elements. (F, Fi) Mesodermally-derived chondrocytes were also recovered in the ceratobranchial of gill arch 5, in close proximity to the label-retaining pectoral girdle and surrounding connective tissue. (G) Schematic summary of pharyngeal and paired fin skeletal elements in the S32 skate embryo, with elements receiving any mesodermal contributions (HM, HM-LPM or LPM) coloured red, and a plot showing the number of embryos observed with mesoderm contributions to the pharyngeal arch and pectoral fin skeleton. In (D), (E) and (F), cartilaginous elements are false-coloured yellow. Scale bars: A, B = 700 µm; Ai = 125 µm; Aii = 15 µm; Bi = 50 µm; Bii = 15 µm; C = 60 µm; Ci = 20 µm; D = 50 µm; Di = 5 µm; E = 30 µm; Ei = 5 µm; F = 60 µm; Fi = 7 µm.