36 Greek Biology 



distinctly visible. The head still is larger than the rest of the 

 body and the eyes larger than the head. At this time also the 

 larger internal organs are visible, as also the stomach and 

 the arrangement of the viscera ; and the vessels that seem to 

 proceed from the heart are now close to the navel. From the 

 navel there stretch a pair of vessels, one [vitelline vein] towards 

 the membrane that envelops the yolk, and the other [allantoic 

 vein] towards that membrane which envelops collectively the 

 membrane wherein the chick lies, the membrane of the yolk 

 and the intervening liquid. . . . About the twentieth day, if 

 you open the egg and touch the chick, it moves inside and 

 chirps ; and it is already coming to be covered with down 

 when, after the twentieth day, the chick begins to break the 

 shell. 5 1 



Aristotle recognized a distinction in the mode of develop- 

 ment of mammals from that of all other viviparous creatures. 

 Having divided the apparently viviparous animals into two 

 groups, one of which is truly and internally and the other only 

 externally viviparous, he pointed out that in the mammalia, 

 the group regarded by him as internally viviparous, the foetus 

 is connected until birth with the wall of the mother's womb by 

 the navel string. These animals, in his view, produce their 

 young without the intervention of an ovum, the embryo being 

 ' living from the first '. Such non-mammals, on the other 

 hand, as are viviparous are so in the external sense only, that 

 is, the young which he considered to arise in this group from 

 ova may indeed develop within the mother's womb and be 

 born alive, but they go through their development without 

 organic connexion with the mother's body, so that her womb 

 acts but as a nursery or incubator for her eggs. It was indeed 

 a sort of accident among the ovipara whether in any particular 

 species the ovum went through its development inside or out- 

 side the mother's body. ' Some of the ovipara ', he says, 



1 Historta animalium, vi. 3 ; $6i a 18. 



