embryos. And here that practice of the poor folks finds its application : " Having hens at home, but no cock, they commit their females to a neighbour's male for a day or two ; and from this short sojourn the fecundity of the whole of the eggs that will be laid during the current season is secured." 1 Not only are those eggs which are still nothing more than yelk and have no albumen, or which exist only as most minute specks in the ovary, but eggs not yet extant, that will be conceived long after- wards, rendered fertile by the same property.
EXERCISE THE THIRTIETH.
Of the uses of this disquisition on fecundity.
This disquisition on the inherent qualities of the egg and the cause of its fecundity, is alike in point of difficulty and subtlety, but of the highest importance. For it was imperative on us to inquire what there was in the conception, what in the semen masculinum, and what in the female fowl, which render these fertile ; and what there is in the fruitful cock which makes him diifer from a bird that is barren. Is the cause identical with that which we have called the vital principle (anima) in the embryo, or it is a certain portion of the vegetative principle ? Because, in order to apprehend the entire cause of generation, it is of much moment that the first cause be understood ; for science is based upon causes, especially first causes, known. Nor is this inquiry less important in enabling us to understand the nature of the vital principle (anima). These questions, indeed, rightly apprehended, not only are Aristotle's opinions of the causes of generation refuted or corrected, but all that has been written against him is easily understood.
We ask, therefore, whether it is the same thing or something different, which in the rudimentary ovum, yelk, egg, cock and hen, or her uterus, confers fruitfulness ? In like manner in what respect does this something agree or differ in each ? Still far- ther, is it a substance whence the fecundating virtue flows?
1 Fabricius, op. cit. p. 37.
