Ossification of distal phalanges and the sixth protrusion in X. tropicalis.
a–d, Bone and cartilage at the distal-most region in the X. tropicalis hindlimb. a, Stage 58. Ossification of the distal phalanx in digit I was detectable (shown by black arrowhead). b, Stage 59. Substantial ossification of the distal phalanges of digits I, II and III could be seen. Ossification of the distal element in the sixth protrusion was initiated (shown by red arrowhead). c, Stage 60. Ossification of the distal phalanges in digits IV and V started to be visible. Note that in all distal elements, ossification is initiated at the distal ends of the structures. d, The froglet showed complete ossification of the distal elements in all of digits I–V and the protrusion. Images are anterior views. Proximal is left and dorsal is top. Scale bars: 200 μm