464 ON GENERATION. 



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. 



