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. 
