Aristotle recognized a distinction in the mode of development of mammals from that of all other viviparous creatures. Having divided the apparently viviparous animals into two groups, one of which is truly and internally and the other only externally viviparous, he pointed out that in the mammalia, the group regarded by him as internally viviparous, the foetus is connected until birth with the wall of the mother’s womb by the navel-string. These animals, in his view, produce their young without the intervention of an ovum, the embryo being ‘living from the first’. Such non-mammals, on the other hand, as are viviparous are so in the external sense only, that is, the young which he considered to arise in this group from ova may indeed develop within the mother’s womb and be born alive, but they go through their development without organic connexion with the mother’s body, so that her womb acts but as a nursery or incubator for her eggs. It was indeed a sort of accident among the ovipara whether in any particular species the ovum went through its development inside or outside the mother’s body. ‘Some of the ovipara’, he says, ‘produce the egg in a perfect, others in an imperfect state, but it is perfected outside the body as has been stated of fish.’[38] Yet though Aristotle regarded fish as an oviparous group, he knew also of kinds of fish that were externally viviparous. It is most interesting to observe, moreover, that he was acquainted with one particular instance among fish in which matters were less simple and in which the development bore an analogy to that of the mammalia, his true internal vivipara. ‘Some animals’, he says, ‘are viviparous, others oviparous, others vermiparous. Some are viviparous, such as man, the horse, the seal and all other animals that are hair-coated, and, of marine animals, the Cetaceans, as the dolphin, _and the so-called Selachia_.’[39] Aristotle tells us elsewhere that a species of these Selachia which he calls _galeos_—a name still used for the dog-fish by Greek fishermen—‘has its eggs in betwixt the [two horns of the] womb; 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, so that, as the egg-substance gets used up, the embryo is sustained to all appearances just as in quadrupeds. The navel-string is ... attached as it were by a sucker, and also to the centre of the embryo in the place where the liver is situated.... Each embryo, as in the case of quadrupeds, is provided with a chorion and separate membranes.’[40] [38] _De generatione animalium_, iii. 9; 758ᵅ 37. [39] _Historia animalium_, i. 5; 489ᵅ 35. [40] _Historia animalium_, vi. 10; 565ᵇ 2.