Aristotle’s description of the "navel-string" linking 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 (from History of Animals; tr. Thompson, revised):

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 side of the womb, and females on the right-hand side, and males and females on the same side together. If the embryo is cut open, then its internal organs (such as the liver) are found to be large and supplied with blood, just as in quadrupeds.

When the egg is now ten days old the chick and all the parts are distinctly visible. The head is still larger than the rest of the body, and the eyes larger than the head, but still sightless. The eyes, if removed about this time, are found to be larger than beans, and black; if the outer membrane is peeled off, there is a white and cold liquid inside, which glitters quite a bit in the sunlight, but there is no hard substance at all. Such is the condition of the head and eyes. At this time also the larger internal organs are visible, along with the stomach and the arrangement of the viscera; and the veins that stretch from the heart are now close to the navel. A pair of veins extends from the navel, one toward the membrane that envelops the yoke—which, by the way, is now liquid—and the other towards the membrane that contains both of the others: namely, the membrane within which the chick lies and the membrane of the yolk, along with the intervening liquid. On the tenth day the white is at the extreme outer surface, reduced in quantity, glutinous, firm, and with a pale color.