ARISTOTLE'S BIOLOGY the womb, and as living in a fuller sense (or with more kinds of life) than could be ascribed to an egg. In the three lower orders of blooded animals, the young developed from an egg; hence these were essentially oviparous, although the egg might hatch within the mother and the young come forth alive, as is the case of certain sharks. Such animals were externally viviparous, yet the young began as an egg and not as a living foetus. In the grounds of this classification there was fundamental error, arising from Aristotle's ignorance of the mammalian egg, and yet much penetrating observation^ the results of which still hold. His work upon the chick of the domestic fowl, and his extraordinary anticipa- tory description of the gestation of certain sharks are examples. In his method of close continuous study of the chick developing with- in the egg, he may have been preceded by the writer of one of the Hippocratic tracts. ^^ " Generation from the egg proceeds in an identical manner with all birds, but the full periods from conception to birth differ. . . . With the common hen, after three days and three nights, there is the first indication of the embryo . . . the heart appears like a speck of
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