GENERATION OF ANIMALS, III. ii. 



this blood-vessel, one to the membrane which sur- 

 rounds the yolk, the other to the chorion-Hke mem- 

 brane which surrounds the animal on all sides ; this 

 one goes round inside the membrane of the shell." 

 Through one of these cords the embryo receives the 

 nourishment from the yolk ; and the yolk increases 

 in bulk, becoming more fluid as it is heated,* since 

 the nourishment, being corporeal, must be avail- 

 able in fluid form, just as it must for plants, and the 

 embryos that are in process of formation, either 

 within the egg or within the uterus, are to begin with 

 living the life of a plant, since their first growth and 

 nourishment they obtain through being fastened on 

 to something. The other umbilical cord extends to 

 the chorion which surrounds the embryo. In the 

 case of the animals that are produced ovipar- 

 ously, we should think of them (a) as having the 

 same relationship to the yolk as the viviparously 

 formed embryos have to the mother, so long as 

 they are within the mother ; for since the nourish- 

 ment of the oviparously formed embryos is not 

 completed within the mother, when they leave her 

 they take a part of her out \\ith them ; (h) as having 

 the same relationship to the outermost — the blood- 

 like — membrane as the other embryos have to the 

 uterus. Also, the eggshell which encloses the yolk 

 and the chorion gives the egg an envelope ana- 

 logous to the uterus : it is as though you were to 

 envelop both a viviparously produced embryo itself 

 and its mother entire. '^ The reason why this is so is 

 that the embryo must be in the uterus, i.e., in contact 

 with the mother. Very well then : in the case of the 

 viviparously produced animals, the uterus is in the 

 mother ; but with the oviparously produced ones 



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