354 ON GENERATION. 



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. 



