Expression of the retinoic acid receptor (rar) gene rar4 during development of the European river lamprey, Lampetra fluviatilis, and of the sea lamprey, Petromyzon marinus. The expression of L. fluviatilis rar4 was characterized by in situ hybridization in embryos at stages (st) 19 (A) and (E), 23 (B) and (F), 25 (C) and (G), as well as 26 (D) and (H). Higher magnifications of the anterior regions of embryos at stages 19, 23, 25, and 26 are shown in (E), (F), (G), and (H), respectively. Cross sections of the embryos at stages 23 (F’) and 25 (G’) are through the anterior spinal cord. The expression of P. marinus rar4 was characterized by in situ hybridization in embryos at stages 19 (I) and (M), 23 (J) and (N), 25 (K) and (O), as well as 26 (L) and (P). Higher magnifications of the anterior regions of embryos at stages 19, 23, 25, and 26 are shown in (M), (N), (O), and (P), respectively. The cross section of the embryo at stage 23 is through the anterior spinal cord (N’) and those of the embryo at stage 25 are through the mouth region (O’), otic vesicle (O”), and posterior pharynx (O'''). All embryos are oriented with anterior to the left. White arrowheads indicate hindbrain expression and black arrowheads expression in the spinal cord. Arrows in (O''') show pharyngeal expression, while arrows in (J), (N), as well as in (O’) highlight mandibular expression. Scale bars for L. fluviatilis: 100 μm for (A), (B), (C), and (D); 50 μm for (E), (F), (G), (H), (F’), and (G’). Scale bars for P. marinus: 500 μm for (I), (J), (K), and (L); 100 μm for (M), (N), (O), (P), (N’), (O’), (O”), and (O”’). Abbreviations: llp, lower lip; tb, tail bud; ulp, upper lip; 1 to 8, pharyngeal pouches 1 to 8.