GENERATION OF ANIMALS, II. iii.-iv. 



same stamp ; they diflPer only by the " more and 

 less," or putting it generally, by excess and de- 

 ficiency."] 



So far as those animals whose nature is more im- IV 

 perfect are concerned,'' as soon as a perfect fetation *■ 

 has been formed, though it is not so far a perfect 

 animal, they expel it. The reasons for this I have 

 already stated. A fetation is perfect by the time it 

 is either male or female. (This applies to those 

 animals whose offspring have this distinction of sex, 

 for there are some which generate offspring that 

 are neither male nor female ; these are the animals 

 which are not themselves produced by male and 

 female parents — not produced in fact as the result 

 of the copulation of a pair of animals. We vriW speak 

 later of the way in which these are generated.) 



The perfect animals, the ones which are intei-nally 

 viviparous, retain \Aithin themselves the animal which 

 is forming, and it remains joined to them until it is 

 brought to birth and expelled. 



With regard to those which are internally oxi- 

 parous in the first stage although they are externally 

 vi\"iparous, the egg, when it has been perfectly 

 formed, in some cases (a) is released, just as it is in 

 the externally o\"iparous animals, and the animal is 

 produced out of the egg inside the female ; in other 

 cases (6), when the nourishment in the egg has been 

 used up, the supply for the creature's perfecting is 

 derived from the uterus ; and that is why the egg 

 is not released from the uterus. This distinguishing 

 feature belongs to the Selachian fishes, which will 

 have to receive special mention later.** 



For the present, however, we must begin first of &*neration 

 all with the animals that come first. These are the '° ^''•p*'*- 



177 



