ON GENERATION. 361 



into froth by the spirits and ejected with force. And even 

 then perchance it is not endowed with equal fecundating force 

 at all times. Neither are all the germs of yelks in the ovary, 

 nor all the eggs in the uterus made fertile at the same instant. 



Now I call that fruitful which, unless impeded by some ex- 

 trinsic cause, attains by its inherent force to its destined encl, 

 and brings about the consequence for the sake of which it is 

 ordained. Thus the cock is called fruitful which has his hens 

 more frequently and surely pregnant, the eggs they lay being 

 at the same time perfect and proper for incubation. 



The hen in like manner is esteemed fruitful which has the 

 faculty of producing eggs, or of receiving and long retaining 

 the virtue of prolific conception from the cock. The cluster of 

 germs and the ovary itself are regarded as prolific when the 

 germs are numerous and of good size. 



The egg in the same way is fruitful which differs from a 

 subventaneous or hypenemic egg, and which, cherished by 

 incubation, or in any other way, does not fail to produce a 

 chick. 



Such an efficient cause consequently is required for the 

 chick, as shall impart the virtue of fecundity to it, and secure 

 it the power of acting as an efficient cause in its turn. Be- 

 cause that, or its analogue at least, by means of which they 

 become prolific, is present in all animals. And the inquiry 

 is the same in each case, when we ask what it is in the egg 

 which renders it prolific, and distinguishes it from a wind 

 egg; what in the vitellary germ and ovary; what in the 

 female; what, finally, in the semen and the cock himself? 

 What, moreover, it is in the blood and punctum saliens, or 

 first formed particle of the chick, whence all the other parts 

 arise with their appropriate structures and arrangements ; what 

 in the embryo or chick itself whereby it becomes more or less 

 robust and agile, attains to maturity with greater or less ra- 

 pidity, and lives with various degrees of health, for a longer or 

 shorter period ? 



Nor is the inquiry very different which goes to ascertain 

 what sex the male and the female, or the cock and the 

 hen, confer upon the prolific egg; and what proceeds from 

 each that contributes to the .perfection or resemblance of 

 the chick, viz., whether the egg, the conception, the matter, 



