Fig. 9. Distribution of mRARß transcripts in vehicle-treated and BMS453-treated wild-type embryos after 24 hours in culture. (A,B) Frontal histological sections through the rhombencephalon. (C,D) Embryos hybridized in toto with the mRARß probe cut sagittally into halves; the left medial side is displayed. aE and pE, anterior and posterior parts of the pharyngeal endoderm, respectively; F, frontonasal process; H, heart; O, otocyst; R4-R7, rhombomeres 4 to 7, respectively. Scale bar in B: 150 µm for A,B.