FERTILITY OF PARENT SPECIES AND HYBRIDS. 85 



duced sterile male but fertile female hybrids. The female hybrids 

 were crossed back to males of both parent types; but the male hybrids 

 remained sterile, although 19 were tested and included i, f , |, and | 

 domestic-blooded males. 



The gayal, Bibos frontalis, has been crossed with the domestic cow 

 and likewise produced fertile female but sterile male hybrids. At least 

 6 ^-gayal bulls were tested and found to be sterile, but 3 out of 9 |- 

 gayal bulls were fertile. 



The gaur, Bihos gaurus, considered a close relative to the gayal, was 

 crossed with the domestic cow. A male hybrid was sterile to cows 

 (although he covered 19), but, strangely enough, he was fertile with 

 his own sisters. 



The banteng, Bibos sondaicus, was crossed with the zebu, Bos indicus, 

 and produced a sterile male. I have been told that the female hybrids 

 are fertile, and regard the sources of information as reliable. 



The bison, Bison americanus, has been reciprocally crossed with 

 domestic cattle, but most successfully when a domestic bull is used. 

 The hybrids, frequently called cattaloes, are sterile males and fertile 

 females. The female hybrids have been crossed back to males of both 

 parent species, thus producing | and f bison (Boyd 1908; Iwanoff 

 1911). The i bison females are fertile, as may be expected. The 

 I bison females have not been fully tested, but are presumably also 

 fertile. The I bison males are not always fertile, for Boyd reports the 

 appearance of but 1 out of 4 tested males. Iwanoff reports a fertile 

 f bison male and supposes, on purely theoretical grounds, that a mating 

 of such a fertile male with a j bison female would result in fertile 

 ^ bison of both sexes. Boyd has more recently reported other fertile 

 hybrid males (Boyd 1914). 



19. THE FERTILITY OF THE MALE HYBRIDS. 

 MATERIALS AND METHODS. 



The first two generations of male hybrids (the ^ and the | wild) were 

 few in number and could be tested thoroughly by mating them to 

 guinea-pigs or to their fertile hybrid sisters. But since the number of 

 hybrids to be tested increased so rapidly in the succeeding generations 

 (see table 72) that facilities were lacking to mate all of them, it became 

 necessary to resort to another method, if any knowledge of their fertility 

 was to be acquired. In testing the fertility of hybrid males by breed- 

 ing, it was necessary to keep them with four of five females for at least 

 4 months. Furthermore, it was found that much time and space were 

 being wasted in trying to prove animals sterile or fertile by a breeding 

 test, when a simple examination of the contents of the epididymis 

 would show immediately whether it was useless to attempt to breed 

 the hybrid. Therefore I decided to test each animal microscopically 



