PRINCIPLES OF LIVESTOCK BREEDING. 29 



The common belief that particular animals have a tendency to 

 produce an excess of males or females has rather more support than 

 the other theories mentioned above. Breeders' of dairy cattle in 

 particular often become discouraged with a bull which seems to sire 

 largely bull calves. It must be remembered in this connection, 

 however, that rather large departures from equality may occur 

 simply by chance. Thus if a coin is tossed 20 times, the best expec- 

 tation is 10 heads and 10 tails, but about once in 40 times a departure 

 as great or greater than 15 heads or tails is to be looked for. Thus 

 a large number of dairy-cattle breeders may be expected to get 15 

 or more bull calves out of 20 calves born. Such a result in one year 

 would not have the slightest effect on the sex ratio in the next. 



However, very extensive experiments with rats, made by Dr. 

 Helen D. King, of the Wistar Institute, have shown that it is possible 

 by selection, accompanied by inbreeding, to produce strains which 

 differ considerably in sex ratio. She obtained 122 males to 100 

 females in the strain selected for male production and 82 males to 

 100 females in the strain selected in the opposite direction. 



The theory that sex is normally determined by the number of 

 chromosomes brought together by the sperm and egg at fertilization 

 does not necessarily mean that this is the only method. There is, in 

 fact, a certain amount of evidence which indicates that under extreme 

 conditions the sex, as determined by the chromosomes, may be 

 reversed. In hybridizing, especially, the normal mechanism seems 

 likely to be upset and a great excess of males or females may be 

 produced. 



THE FREEMARTIN. 



An interesting case of incomplete reversal of sex has recently been 

 solved and may be mentioned in this connection. It has long been 

 known that a heifer calf, born as a twin with a bull, is, in 8 or 9 cases 

 out of 10, sterile. Such a heifer is called a " freemartin." The cause 

 of this phenomenon has recently been worked out independently by 

 Tandler and Keller in Germany and by Prof. F. R. Lillie, of the 

 University of Chicago. They found that the blood systems of cattle 

 twins usually grow together. When both twins were females or 

 both males, no harm resulted. When a female was the twin of a male 

 the development of the former appeared wholly normal in the few 

 cases in which the blood systems remained separate. In all the 

 cases in which the blood systems were connected, the female showed 

 an abnormal development, intermediate between that of a female 

 and a male. It appeared that the male embryo secreted some 

 substance into the blood which tended to reverse the sexual develop- 

 ment of the female embryo. 



