Diseases of the Reproductive Organs 



275 



only two tenths of 1 per cent of the eggs are double-yolked. 

 The ratio of double to single yolked eggs is less than twice 

 as high as the ratio of twin to single births in the human 

 family. 



All birds are not equally likely to lay double-yolked eggs. 

 In fact the great majority of birds never lay anything but 

 single-yolked eggs. There are, however, birds which possess 

 a tendency to lay double-yolked eggs. Such an individual 

 may produce several such eggs. It has been further found 



Fig. 58. — Tripk'-.\-( 



(Original.) 



that a bird which possesses the tendency to lay double- 

 yolked eggs is not equally likely to produce them at any age. 

 She is most likely to produce them when she is young. 

 Eighty per cent of all the double-yolked eggs produced by 

 the Station flock are produced by birds less than eight months 

 old. We have only a very few records of birds which have 

 laid double-yolked eggs after their first adult molt. 



It has been usually supposed that double-yolked eggs 

 are caused by the simultaneous entrance of two yolks into 

 the egg tube and the consequent common passage of the two 



