Aristotle's biology packed full of a multitude of facts, in no one of which did Johannes Miiller discover a flaw. The subject is technical, but the gist of the matter is this: that among the Selachians (as, after Aristotle, we still sometimes call them) there are many diversities in the structure of the parts in question, and several distinct modes in which the young are brought forth and matured. For in many kinds an egg is laid, which eggs, by the way, Aristotle de- scribes with great minuteness. Other kinds do not lay eggs, but bring forth their young alive, and these include the Torpedo and numerous sharks or dogfish. The egg-shell is in these cases very thin, and breaks before the birth of the young. But among them there are a couple of sharks, of which one species was within Aristotle's reach, where a very curious thing happens. Through the delicate mem- brane, which is all that is left of the egg-shell, the great yolk-sac of the embryo becomes con- nected with the parental tissues, which infold and interweave with it; and by means of this tempKDrary union the blood of the parent be- comes the medium of nourishment for the young. And the whole arrangement is physio- logically identical with what obtains in the
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