Fig. 6.

(a) Scanning electron micrograph illustrating the distal end of the fenestrated sleeve-like tissue fold subtended by the foramen ovale in the fetus of an incompletely identified species, possibly sperm whale (Physeter catodon). The tissues labelled AW are sections of atrial wall. Scale bar = 1 mm. (b) Scanning electron micrograph illustrating the network of threads at the distal end of the fenestrated sleeve-like tissue fold subtended by the foramen ovale in an unidentified ‘baleen whale’ fetus. Scale bar = 1 mm.