Aristotle's description of the "navel-string" links embryo to placenta in a species of dog-fish called the "smooth shark." Disbelieved for nearly two millennia, its existence was finally confirmed by researchers in the middle of the nineteenth century.

The so-called smooth shark has its eggs in between the wombs, like the dog-fish; these eggs shift into each of the two horns of the womb and descend, and the young develop with the navel-string attached to the womb.... The navel-string is long and adheres to the underside of the womb—each string being attached as it were by a sucker—and also the center of the embryo in the place where the liver is situated. . . . When young, the embryo has its head pointing upwards, but downwards when it becomes strong and has completed its growth. Males are generated on the left-hand...