(A) Four-chamber view of the ventricular base, shows that the membranous septum is rich in myocardium. (B) Tbx3 is expressed in the atrioventricular canal myocardium and in the crest of the ventricular septum (red arrowheads). (C) Expression of Tbx3 is deep in the ventricular septum. (D) Transverse section, showing the ventricular septum (framed) and the membranous septum, again rich in myocardium (red arrowhead). (E) Tbx3 is, like in B-C, expressed deep in the ventricular septum. (F-H) Shows a higher magnification of the area shown in E, in neighboring sections, to show an overlap of expression of Tbx3 (F) and Gja5 (G). This part of the Tbx3 expressing domain did not express atrial myosin (Myh6, white arrowhead) (H). (I-K) In contrast, the Tbx3 expressing myocardium of the left atrioventricular junction (I), did express atrial myosin (K) and showed little if any expression of Gja5 (J). This suggests that the Tbx3 identified bundle in the alligator, is ventricular as in mammals (Aanhaanen et al., 2010).