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]
^
[58]
