O MAINE AGRICULTURAL EXPERIMENT STATION. 



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 yolks 

 through the duct. A careful study of the structure of all the 

 double-yolked eggs produced by the Station flock shows that 

 in only a small per cent (i6) of the cases have the two yolks 

 passed the entire length of the duct together. In such cases 

 the two yolks are enclosed in a common thin layer of white 

 membrane, the chalazal membrane, and have only one pair of 

 chalazae. They also have common albumen envelopes as well 

 as a common egg membrane and shell. 



Since the formation of each egg part (chalazal membrane 

 and chalazae, thick albumen, egg membrane, and shell) is con- 

 fined to a particular part of the oviduct, a study of the number 

 of the secondary parts which are common to the two yolks of 

 a double-yolked egg shows the level of the duct where the two 

 yolks came together. Such a study carried out on all double- 

 yolked eggs produced by the large flock of birds owned by this 

 Station shows that the twO' yolks unite at every level of the 

 duct from the mouth of the funnel to the very end of the albu- 

 men secreting portion. It shows further that the number of 

 eggs of any given structure observed is exactly equal to .the 

 number expected on the assumption that the union of the two 

 yolks occurs indiscriminately at every level of the duct from 

 the mouth of the funnel to the beginning of the isthmus or egg 

 membrane secreting portion. When two eggs unite after the 

 first egg has received its membrane the result is two eggs at the 

 same time. 



The structure of the egg has shown us that in a majority of 

 cases the two yolks of a double-yolked egg have not passed the 

 entire length of the duct together. On a moment's reflection 

 we see that there was never any a priori reason for the assump- 

 tion that the cause for the production of a double-yolked egg 

 was necessarily the simultaneous discharge of two yolks from 

 the ovary into the oviduct or egg tube. The only condition 



