Further, the definition of an egg, as given by Aristotle/ is perfectly applicable to a conception : " An egg," he says, " is that the principal part of which goes to constitute an animal, the remainder to nourish the animal so constituted." Now the same thing is common to a conception, as shall be made to appear visibly from the dissection of viviparous animals.
Moreover, as the chick is excluded from the egg under the influence of warmth derived from the incubating hen or ob- tained in any other way, even so is the foetus produced from the conception in the uterus under the genial warmth of the mother's body. In few words, I say, that what oviparous animals supply by their breast and incubation, viviparous ani- mals afford by their uterus and internal embrace. For the rest, in all that respects the development, the embryo is produced from the conception in the same manner and order as the chick from the egg, with this single difference, that whatever is re- quired for the formation and growth of the chick is present in the egg, whilst the conception, after the formation of the em- bryo, derives from the uterus of the mother whatever more is requisite to its increase, by which it continues to grow in com- mon with the foetus. The egg, on the contrary, becomes more and more empty as the chick increases; the nutriment that was laid up in it is diminished; nor does the chick receive aught in the shape of new aliment from the mother ; whilst the foetus of viviparous animals has a continued supply, and when born, moreover, continues to live upon its mother's milk. The eggs of fishes, however, increase through nourishment obtained from without; and insects and crustaceous and molluscous animals have eggs that enlarge after their extrusion. Yet are not these called eggs the less on this account, nor, indeed, are they therefore any the less eggs. In like manner the concep- tion is appropriately designated by the name of ovum or egg, although it requires and procures from without the variety of aliment that is needful to its growth.
Fabricius gives this reason for some animals being oviparous, for all not producing living offspring : " It is," he says, " that eggs detained in the uterus till they had produced their chicks would interfere with the flight of birds, and weigh them
1 Hist. Anim. lib. i, cap. v.
