Fig. 5.

(a) Scanning electron micrograph illustrating the network of threads at the distal end of the fenestrated sleeve-like fold of tissue subtended by the foramen ovale in a bowhead whale (Balaena mysticetus) fetus. The sleeve has rolled upon itself. Scale bar = 1 mm. (b) Scanning electron micrograph illustrating the threads at the distal end of the unfenestrated tunnel-like fold of tissue from the foramen ovale in a north atlantic minke whale (Balaenoptera acutorostrata) fetus. Scale bar = 1 mm. (c) Scanning electron micrograph illustrating the network of threads at the distal end of the fenestrated sleeve-like fold of tissue subtended by the foramen ovale in a fin whale (Balaenoptera physalus) fetus. Scale bar = 1 mm. (d) Scanning electron micrograph illustrating the partially autolysed threads at the distal end of the fenestrated sleeve-like fold of tissue subtended by the foramen ovale in a humpback whale (Megaptera novaeangliae) fetus. Scale bar = 1 mm.