The history of the discovery of germinative foliations in the embryonic development of vertebrates forms one of the most impressive of human documents. In 1700 the theory of _pre-formation_ was vigorously upheld amongst the many ideas relating to generation: that is to say, it was believed that the germs contained little organisms completely formed which would eventually unfold and increase the parts of infinitesimal dimensions which were packed one within the other. This theory applied to every living creature, animal, vegetable, and human. It had led, by its own logical development, to the more far-reaching theory of "mutual inclusion"--that is, the doctrine that, as all living organisms are _pre-formed_, they must of necessity all have existed from the Creation, the one included, or wrapped up, in the other. All humanity must have lain in the ovaries of Eve. When in 1690 Leuwenhoek discovered spermatozoa by the aid of the microscope, the idea was evolved that each male cell contained a complete microscopic man, the _homunculus_; and then it was announced that not Eve, but Adam had contained all humanity within himself. Hence the two contradictory theories which in the eighteenth century kept their adherents sharply divided, the theories of the ovulists and those of the animalculists, and the dispute seemed to offer little hope of a possible decision. The names of famous scientists and philosophers were associated with these dissensions, those, for instance, of Spallanzani and of Liebnitz, who applied the principles of generation even to the soul. "Thus I should think," said Liebnitz, "that the souls which will one day become human souls, were present in the germ; that they have always existed as organized bodies in their progenitors from Adam onwards--that is, from the beginning of things." [8]