Transverse section through the region of the heart in a rabbit embryo of nine days. X 80. (Kölliker.) j, j. Jugular veins. ao. Aorta. ph. Pharynx. som. Somatopleure. bl. Proamnion. ect. Ectoderm. ent. Entoderm. p. Pericardium. spl. Splanchnopleure. ah. Outer wall of heart. ih. Endothelial lining of heart. é. Septum between heart tubes.