ON GENERATION. 447 



chick finds the analogue of both kinds of food in the egg. 

 So that whilst in viviparous animals the uterus exists within 

 the parent, in oviparous the parent may rather be said to exist 

 within the uterus (the egg). For the egg is a kind of exposed 

 and detached uterus, and in it are included in some sort vicarious 

 mammae. The chick in the egg, I say, is first nourished by 

 albumen, but afterwards, when this is consumed, by the yelk 

 or by milk. The umbilical vascular connexion with the albu- 

 men, therefore, when this fluid is used up, withers and is in- 

 terrupted when the abdomen comes to be closed, and be- 

 fore the period of exclusion arrives, so that it leaves no trace 

 of its existence behind it : in viviparous animals, on the con- 

 trary, the umbilical cord is permanent in all its parts up to the 

 moment of birth. The other canal that extends to the vitellus, 

 however, is taken up along with this matter into the abdomen, 

 where being stored, it serves for the support of the delicate 

 foetus until its beak has acquired firmness enough to seize 

 and bruise its food, and its stomach strength sufficient to com- 

 minute and digest it ; just as the young of the viviparous ani- 

 mal lives upon milk from the mammae of its mother, until it is 

 provided with teeth by which it can masticate harder food. For 

 the vitellus is as milk to the chick, as has been already said ; 

 and the bird's egg, as it stands in lieu both of uterus and 

 mammas, is furnished with two fluids of different colours, the 

 white and the yelk. 



All admit this distinction of fluids. But I, as I have already 

 said, distinguish two albumens in the egg, kept separate by an 

 interposed membrane, the more external of which embraces the 

 other within it, in the same way as the yelk is surrounded by 

 the albumen in general. I have also insisted on the diverse 

 nature of these albumens ; distinguished both by situation and 

 their surrounding membranes, they seem in like manner calcu- 

 lated for different uses. Both, however, are there for ends 

 of nutrition, the outermost, as that to which the branches of 

 the umbilical veins are earliest distributed, being first con- 

 sumed, and then the inner and thicker portion ; last of all the 

 vitellus is attacked, and by it is the chick nourished, not only 

 till it escapes from the shell but for some time afterwards. 



But upon this point we shall have more to say below, when 

 we come to speak of the manner in which the foetuses of 



