70 MAYN1E R. CURTIS. 



there meets another, and the two come down together, a double 

 egg (ovum in ovo) will be formed. If, however, an egg which has 

 not received its shell membrane is carried back up the duct and 

 meets another coming down the result \vill be some form of 

 double-yolked egg, the particular sort depending upon the rela- 

 tive development of the two eggs when they unite. The condi- 

 tions under which an egg without a shell membrane would 

 find another egg in the duct even if returned by antiperistalsis 

 are not usual, since a second yolk does not normally enter the 

 duct until some time after its predecessor has received a shell 

 membrane. Such conditions might be brought about either by 

 the entrance of a second yolk at an abnormally short interval 

 after the first or by the first egg remaining stationary for some 

 time or returning slowly. 



It has been shown that the conditions necessary for the forma- 

 tion of a double-yolked egg (or a double egg) are that the two 

 eggs unite in the oviduct and then proceed through the duct 

 together. In all cases where the structure of the egg shows that 

 the two components have not passed the entire length of the duct 

 together the passage of the second egg must have been more rapid 

 than that of the first for should the two eggs continue to move 

 through the duct at the same rate they would only unite in the 

 cases where their entrance was practically or absolutely simul- 

 taneous. In the preceding paragraphs the possibility of the 

 delay or return of the first egg has been discussed. One further 

 possibility is that the second yolk may pass through the duct at 

 an abnormally rapid rate. 



It is thus possible that double-yolked eggs do not always 

 represent a simultaneous or abnormally rapid succession of two 

 ovulations since a yolk may sometimes remain in the oviduct, or 

 body cavity, until the time normal for a second ovulation. There 

 are some cases of double-yolked egg production, however, which 

 can only be explained by the occurrence of two or more ovulations 

 at unusually short intervals. Such cases are seen when double- 

 yolked eggs are produced on successive days, and when a double- 

 yolked egg is laid after a long series of normal daily eggs. Several 

 records of this kind have been obtained from the station flock. 



