920 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



The locality in which fertilization takes place may be either the 

 ovary, as is shown by the occurrence of abdomidal pregnancy, the Fal- 

 lopian tube, or the cavity of the uterus itself. The manner in which the 

 spermatozoa obtain access to the ovum is not capable of clear demon- 

 stration. In the case of the ruminants, as has been seen, there are reasons 

 for believing that the spermatozoa are deposited directly within the 

 cavity of the uterus. In other animals, where in all probability this does 

 not take place, the entrance of the spermatozoa into the uterus may be 

 accomplished by the suction which results from the relaxation of the 

 uterus as this condition of erection and engorgement passes off, and it is 

 a matter of common experience that the introduction of semen even 

 within the vestibule of the vagina is frequently sufficient to produce fer- 

 tilization. It is probable that the vibration of the cilia is concerned in 

 causing their movement to meet the ova. It must not be forgotten, how- 

 ever, that within the uterus, and especially within the Fallopian tubes, the 

 motion of the ciliated epithelium is normally in the opposite direction 

 to that in which the spermatozoa must move to reach the ovary, so that 

 while this ciliary motion would facilitate the descent of the ovum, it must 

 offer an obstacle to the ascent of the spermatozoa. 



Nothing absolutely definite is known as to the manner in which 

 the bringing together of the male and female spermatic elements is 

 accomplished. It is only clearly known that the spermatozoa in some 

 way possess the power of rapidly ascending through the internal sexual 

 organs of the female. Spermatozoa have been found within the uterine 

 cavity of the dog a quarter of an hour after the act of copulation, and in 

 the Fallopian tube, even at its abdominal end, within one or two hours. 



The spermatozoa retain their power of fertilizing the ovum, as indi- 

 cated by the persistence of their power of movement, for as long as eight 

 days while within the body of the female, a period which is abundant to 

 permit the ripening and discharge of an ovum and its contact with the 

 spermatozoa. Even if copulation takes place in the intervals between 

 the maturing of the ova, fertilization may, nevertheless, take place. 



As a rule, fertilization of ova is only possible from the contact with 

 spermatozoa from the same species ; } r et in certain instances fertilization 

 may take place between different species of the same genus of animals, 

 as, for example, between the horse and the ass, between the dog and the 

 wolf, between the dog and the fox, between the lion and the tiger, and 

 between the rabbit and the hare. The results of such impregnation are 

 spoken of as hybrids, and possess characteristics midway between the 

 two species. The products of such impregnation are, as a rule, barren 

 from arrested development of their genital organs (testicle and ovary). 



The impregnated ovum, whatever be the locality in which impreg- 

 nation takes place, is borne to the interior of the uterus, there to 



