The ppt;ntl and kny;ntl double mutants exhibit synergistic tail elongation defects. At the eight-somite stage, the AP axis of the wild-type embryo (A) is elongated such that the head and tail almost touch. This distance is greater (arrows) in ppt (B), and kny (C) mutants exhibiting shortened AP axes. In ntl (D) mutants, the AP axis is only slightly shorter at this stage, whereas ppt;ntl (E) and kny;ntl (F) double mutants are shorter than the individual mutants. At 18 somites, the tail extends beyond the yolk tube (*) in wild-type (G) embryos and is shorter in ppt (H), kny (I) and ntl (J) mutants. At this stage, the tail has not extended in ppt;ntl (K) and kny;ntl (L) double mutants. (A-L) Lateral views. Scale bars=100 μm.