276 Diseases of Poultry 



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 very small per cent of the cases (about 

 16 per cent) have the two yolks passed the entire length of 

 the duct together. In such cases the two yolks are inclosed 

 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 common egg 

 membrane and shell. 



Since the formation of each egg part (chalazal membrane 

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

 confined to a particular part of the oviduct, a study of the 

 number of 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 albumen secreting portion. It shows 

 further that the number of eggs of any given structure ob- 

 served is exactly equal to the number expected on the assump- 

 tion 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 assumption that the cause for the production of a 

 double-yolked egg was necessarily the simultaneous dis- 

 charge of two yolks from the ovary into the oviduct or 

 egg tube. The only condition necessary for two yolks to 



