ARISTOTLE AS A BIOLOGIST 21 



Aristotle's reach, where a very curious thing happens. 

 Through the delicate membrane, which is all that is left 

 of the eggshell, the great yolk-sac of the embryo becomes 

 connected with the parental tissues, which infold and inter- 

 weave with it ; and by means of this temporary union the 

 blood of the parent becomes the medium of nourishment 

 for the young. And the whole arrangement is physio- 

 logically identical with what obtains in the 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 yaAeo? Aetos of Aristotle, that this singular 

 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 

 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 important fact 

 which the Philosopher had studied and explained. In* 

 the seventeenth century, the anatomist Steno actually 

 rediscovered the phenomenon, in the yaAeo? Aaos, the 

 Mustelus laevis itself, but he was unacquainted with 

 Aristotle. And the very fact was again forgotten until 

 Johannes Miiller brought it to light, and showed not only 

 how complete was Aristotle's account, but how wide 

 must have been his survey of this class of fishes to enable 



