THE LIMITS OF HEREDITY 203 



twins as compared with twins which are male and 

 female, and this might be due to the occurrence of 

 monozygotic twins. But in no case is this difference 

 greater than twice the probable error, so it really 

 furnishes no eyidence in fay our of the occurrence of 

 monozygotic twins in cattle. 



Lush (1922) describes a pair of twin Jersey heifers 

 which he thinks are identical. They both haye a 

 moderate notch (see p. 135) in their ears; their noses, 

 tongues, switches, and general shade of body colour 

 are the same; and they each haye a single spot on 

 the right hind foot. This indicates that monozygotic 

 twins occasionally occur in cattle. It is hardly 

 likely that both parents would be homozygous for 

 all these characters. 



In pigeons, two pairs of female monozygotic twins 

 haye been described (Riddle, 191 8). They were both 

 deriyed from eggs with yery large yolks, but not 

 " double-yolked," the embryos haying a single 3^olk 

 sac. It is belieyed that they were formed by the 

 separation of the first two blastomeres in the fertilised 

 egg, this separation being a result of the yery large 

 initial size of the e.gg. 



