Page:The Works of William Harvey (part 1 of 2).djvu/454

 or in an oven, far from the bursa of the parent hen, is still quickened and made to produce an embryo.

The same difficulty still remains, I say : how or in what way is the semen of the cock the " efficient " of the chick ? It is in no wise removed by invoking the irradiation of a spiritual sub- stance. For did we even admit that the semen was stored in the bursa, and that it incorporated the embryo from the cha- lazse by metamorphosis and irradiation, we should not be the less deeply immersed in the difficulty of accounting for the formation of all the internal parts of the chick. But these notions have already been sufficiently refuted by us.

Wherefore, in investigating the efficient cause of the chick, we must look for it as inhering in the egg, not as concealed in the bursa; and it must be such, that although the egg have long been laid, be miles removed from the hen that pro- duced it, and be set under another hen than its parent, even under a bird of a different kind, such as a turkey or guinea- fowl, or merely among hot sand or dung, or in an oven con- structed for the purpose, as is done in Egypt, it will still cause the egg to produce a creature of the same species as its parents, like them, both male and female, and if the parents were of different kinds, of a hybrid species, and having a mixed re- semblance.

The knot therefore remains untied, neither Aristotle nor Fabricius having succeeded even in loosening it, namely: how the semen of the male or of the cock forms a pullet from an egg, or is to be termed the " efficient" of the chick, especially when it is neither present in, nor in contact with, nor added to the egg. And although almost all assert that the male and his semen are the efficient cause of the chick, still it must be ad- mitted, that no one has yet sufficiently explained how it is so, particularly in our common hen's egg.