GREEK BIOLOGY AND MEDICINE 



higher animals, the mammals, or warm-blooded 

 vivipara. It is true that the yolk-sac is not 

 identical with that other embryonic membrane 

 which comes in the mammals to discharge the 

 function of which I speak; but Aristotle was 

 aware of the difference, and distinguishes the 

 two membranes with truth and accuracy. 



" It happens that of the particular genus of 

 sharks to which this one belongs, there are 

 two species differing by almost imperceptible 

 characters; but it is in one only of the two, 

 the 7aXe6s Xetos of Aristotle, that this singu- 

 lar phenomenon of the placenta vitellina is 

 found. It is found in the great blue shark 

 of the Atlantic and the Mediterranean; but 

 this creature grows to a very large size before 

 it breeds, and such great specimens are not 

 likely to have come under Aristotle's hands. 

 Cuvier (i 769-1832) detected the phenomenon 

 in the blue shark, but paid little attention to it, 

 and, for all his knowledge of Aristotle, did 

 not perceive that he was dealing with an im- 

 portant fact which the Philosopher had studied 

 and explained. In the seventeenth century, 

 the anatomist Steno (1638-86) actually re- 

 discovered the phenomenon, in the yaXeos 

 Xetos, the Mustelus laevis itself, but he was 



[58] 



^ 



